At a freshly launched webshop, invoicing is rarely a serious challenge. With a few orders a day, creating and sending the documents can be handled manually, but at higher volumes a completely different system is needed. With fifty, a hundred or even several hundred orders, invoicing has to work quickly, accurately and with as little manual intervention as possible.
At that point the most important question is no longer which invoicing program produces a nice PDF, but how well it can work together with the webshop, the payment system, the bookkeeping and the tax authority solutions of the given country. The larger the order volume, the more important it becomes not to have to record the same data separately in multiple systems.
International sales bring further tasks, since you can encounter different rules and systems in each country. In Hungary, for example, Billingo and the NAV Online Invoice system are dominant, while in Serbia a key element of electronic invoicing is E-Faktura, officially known as the Sistem elektronskih faktura (SEF).
From Sellvio's perspective, electronic invoicing therefore isn't simply about wiring up an external service. The goal is for the data already available at the time of the order to flow automatically into the right invoicing system, and for the process to keep working reliably even at higher volumes or when serving multiple countries.
Invoicing is now part of the webshop's infrastructure
Many merchants still treat invoicing as a separate administrative task: an order comes in, someone opens the invoicing program, copies the data across, creates the document and sends it to the customer. This can work at lower volumes, but every manual step increases the chance of error - a misspelled name, a wrong tax number, an incorrect payment method, or simply a forgotten invoice.
In a well-built webshop, invoicing is therefore no longer a standalone process. The invoice is generated from the same data the customer already provided during the order, so there's no need for redundant data copying, and the risk of administrative errors is significantly lower.
In Sellvio's system, webshop orders can be connected to the supported invoicing and ERP systems. Both Hungarian and Serbian solutions are available, though it's important to bear in mind that the individual integrations can differ by country.
'Invoicing without borders' doesn't mean a universal invoicing program
In international sales, trying to use a single invoicing system in every country isn't necessarily the best solution. Tax, invoicing and electronic data reporting requirements can differ significantly from market to market.
That's why Sellvio builds on different integrations per country. This illustrates well how to think about international operations: rather than forcing a single invoicing tool onto every market, you build a webshop infrastructure capable of connecting to the financial and invoicing systems used in each country.
Electronic invoice, online invoicing software and NAV data reporting: not the same thing
These concepts are worth keeping separate, because issuing an invoice with an online invoicing program doesn't necessarily make it an electronic invoice in the legal sense. In Hungary, moreover, the online invoice data reporting owed to NAV is a separate question again.
Under NAV's rules, data must be reported on the invoices, amendment and cancellation documents that fall under the invoicing rules of the Hungarian VAT Act. This happens through the Online Invoice system.
For electronic invoices, you must additionally ensure the authenticity of the invoice's origin, the integrity of its content and its legibility throughout the retention period. These are no longer mere technical details - they are fundamental requirements of compliant electronic invoicing. So electronic invoicing means much more than sending the invoice to the customer as a PDF.
Billingo and Sellvio
Billingo is one of the well-known online invoicing solutions among Hungarian e-merchants, and it can be connected via API to webshops, CRMs and other business systems. With the current REST API 3.0, invoice issuing, among other things, can be automated.
In a webshop environment, this is precisely the point. What matters isn't how quickly you can create an invoice by hand in a few clicks, but whether you need to re-enter the data already provided at the time of the order at all.
How does all this work in Sellvio?
For the Billingo integration you need to configure, among other things, a V3 API key, an invoice block and a payment deadline. After that, invoicing can be initiated directly from the order administration interface.
The system passes the order data needed to create the invoice - such as the payment method, delivery date, payment deadline, rounding or comments - to Billingo. The created document then also appears in the Billingo account and is available in PDF format.
The real advantage, however, isn't the PDF itself, but the fact that data already available in the webshop doesn't need to be processed manually a second time. That's what turns the invoicing program from a simple administration tool into a genuine e-commerce integration.
Currency, language and foreign customers
We can be certain that in international sales, questions quickly surface that are less prominent in a domestic webshop. These include foreign currency invoicing, the language of the invoice, handling different payment methods, and how an order gets modified or cancelled. It's very important to clarify these before the webshop even launches.
Billingo supports invoicing in multiple currencies, and with certain integrations it's even possible to issue multilingual invoices. However, an international invoicing process doesn't become compliant just because the system is technically able to issue an invoice in euros. The applicable VAT rate, the use of OSS, the invoicing obligation and the rules of the given destination country always depend on the business's specific operations. Technology can automate a correctly designed process, but it will carry a faulty configuration forward with the same consistency - so you must always have solid answers to the financial questions.
Before any serious foreign expansion, it's worth having your invoicing and tax settings reviewed by an accountant or tax advisor as well.
A slightly different world awaits in Serbia: E-Faktura and SEF
If you also sell in Serbia, you'll soon come across the terms E-Faktura and SEF. It's important to see, though, that this isn't the same category as, say, Billingo: while the latter is a commercial invoicing and finance platform, the Serbian Sistem elektronskih faktura (SEF) is a state-run electronic invoicing system.
SEF serves for sending, receiving, processing and storing electronic invoices, and it can be connected to external systems via an API. This also shows how invoicing's role is changing: the invoice is less and less a simple document, and more structured data that various systems process automatically.
Sellvio provides a direct E-Faktura/SEF integration for Serbian webshops. Setting up the connection requires the API key created after SEF registration, after which the order data can be transmitted to the system in structured form. Such data can include the product name, unit price, quantity, VAT content, delivery date, payment deadline or bank account. This is exactly why it's important for the webshop's order data to be properly structured - whatever the invoicing system asks for has to arrive automatically from somewhere.
For invoicing via the SEF integration, the customer must have a tax number and be registered in the SEF system. This illustrates well that a B2B webshop's invoicing process can be considerably more complex than a simple consumer order.
An order actually travels between multiple systems
From all this, the customer usually only notices that they click the Place Order button. In the background, though, a far more complex process can kick off:
Webshop → payment → order management → invoicing → bookkeeping → logistics → tax data reporting.
The value of a modern webshop platform is therefore measured not only in its templates or product page design, but also in which external business systems it can work with. Among Sellvio's integrations, alongside Billingo and E-Faktura, you'll find several further invoicing and ERP solutions, such as Számlázz.hu, Minimax, Calculus, Business Navigator or Dyntell. This can be especially important if you sell in multiple countries and don't use the same backend system in every market. Different invoicing or ERP systems can thus operate in different countries, while sales continue to be managed from the same webshop platform.
What to watch out for before wiring up invoicing
The technical integration is only worth building once the invoicing process itself is clear. You need to clarify in advance, for example, when the invoice should be created: as soon as the order comes in, after payment, or only when the parcel is dispatched. It's equally important to define how the system should handle cash-on-delivery orders, cancellations and refunds, since these are part of e-commerce's everyday operation. When selling in multiple countries, it's also worth deciding right at the start which invoicing system will be used in each market. It also matters where the primary data source lives. The customer's name, address, tax number and the order total should preferably not be kept in multiple systems in differing formats.
The more precisely these rules are defined, the easier it becomes to automate the entire invoicing process later.
What happens when you get 200 orders a day instead of 2?
Manual invoicing can seem manageable for a long time at low volumes, but with growth it very quickly becomes a bottleneck. If creating a single invoice takes just two minutes, a few orders a day means only a few minutes of work - but two hundred orders already mean many working hours. At order volumes in the thousands, manual invoicing would practically require a dedicated administration team. This shows clearly that automation isn't merely a convenience feature, but one of the technical preconditions of growth.
In a well-built system, growth in order volume doesn't come with a proportional growth in administration. That's exactly what allows the webshop to serve higher traffic efficiently.
Europe is heading in this direction too
The role of electronic invoicing is expected to keep strengthening in the coming years. In 2025, the European Union adopted the VAT in the Digital Age (ViDA) package, which reshapes the EU VAT system in several stages. Under the current schedule, from 1 July 2030 new digital reporting requirements built on e-invoicing will take effect for cross-border B2B transactions.
Behind the changes are not only regulatory but also economic considerations. According to the European Commission's calculations, a digital reporting system built on e-invoicing could cut losses from VAT fraud by up to EUR 11 billion a year, while businesses' administrative and compliance costs could fall by more than EUR 4.1 billion.
Electronic invoicing is thus less and less a standalone technological solution, and increasingly an integral part of Europe's tax and corporate infrastructure.
The webshop of the future moves data, not documents
Modern e-commerce systems operate less and less as standalone applications. The webshop handles the order, the payment provider reports back the transaction status, the logistics system tracks the parcel, and the invoicing tool relies on this data to produce the required document.
The goal is that the same information never has to be re-entered in multiple systems. This requires a webshop platform that doesn't try to replace every external business system, but is able to cooperate with them reliably. In Hungary this can describe, for example, the connection between Sellvio and Billingo; in Serbia, linking Sellvio with E-Faktura/SEF; and in other markets, the integration of a local invoicing or ERP system.
So the real advantage comes not from a single feature, but from the fact that the data attached to an order can travel through the entire process without unnecessary manual intervention.
Don't let invoicing become the bottleneck of growth
In a well-run webshop, invoicing must not turn into a separate administrative task for every single order. The customer provides the data, the order enters the system, and from there technology should take over as much of the work as possible.
When choosing a webshop platform, it's worth looking not only at how product pages can be built or how quickly categories can be managed. At least as important is what happens after the order, and how far invoicing, payment, logistics or the ERP connection can be automated.
Sellvio offers multiple invoicing and ERP integrations, including a Hungarian Billingo connection and a Serbian E-Faktura/SEF connection. This means that even for a webshop operating in several markets, you can build a backend that adapts to each country's financial processes.
Ultimately the goal is simple: as your sales grow, the work spent on invoicing shouldn't grow with them.