At Negative Epsilon we've been doing invoicing for 5 years now. We started with consulting projects for some clients back in 2021 and launched our own first product which required invoicing in 2022.

Since then, almost every client project or product we’ve built has required some form of invoicing: billing our own customers, enableing our clients to handle complex invoicing flows, or even to intermediating as a platform that issues invoices on behalf of its merchants.

We've dealt with some ugly scenarios like cross-border invoices where services and goods were mixed, invoicing to governments across national and international networks, suddenly changing tax rate classifications...

But our main lesson has been much less dramatic: for 80% of business activity, invoicing is a boring and repetitive process.

If you're mostly in a single industry, and operating in a single market, then most of your invoices will look very similar. You speak to your accountant once to know which are the taxes you should include in your invoices, and then issue a thousand invoices that look exactly the same over the next few years.

That's what we built FiscalRail for.

FiscalRail is a small, opinionated invoicing API designed to cover the common 80% of invoicing without forcing every integration to absorb the complexity of the remaining 20%.

We're starting with Spain, but have already chosen not to support every single invoicing scenario allowed by Hacienda. Special tax regimes, IGIC, IPSI, special taxes on regulated goods... those are all out of scope for now. Instead, we've chosen to support just the three standard IVA rates (21%, 10% and 4%), IRPF withholding and the special reverse charge. We're sure that won't be enough for everyone but by keeping a narrow scope, we'll be able to keep the workflows we do support simpler and more reliable.

A narrower scope does not mean cutting corners on the underlying infrastructure. FiscalRail is built around immutable issued invoices, atomic numbering, idempotent operations, realistic test accounts, corrections that preserve invoice history, webhook delivery, and professionally rendered tagged PDFs. Those are all components anyone would ask of an invoicing API but in our experience almost no solution exists out there that provides all those without breaking the bank.

This last point is important to us. Being cheap, more specifically, significantly cheaper than the cheapest competitor in our space is also one of our objectives. We believe invoicing is something that should be done right from the start, even at very low volumes, which is why we're launching without monthly fees. live usage is billed per successful operation, while all test usage is free.

FiscalRail itself is new, but it incorporates what we’ve learned from five years of implementing real invoicing workflows. It’s ready for early integrations today, and we have plenty more planned.

Get your API key and issue your first test invoice for free.