[Xylakant]

pay once for something that is fundamentally tied to regularly recurring costs for the vendor is unsustainable. It's a model that works well for desktop software or something self-hosted where the cost of running (and potentially maintaining) occurs on the buyer side.

[vanyaiam]

Im not going to reach true scale here mate. Even if i create 200.000 invoices a day, the cost will be near 0.

[Xylakant]

Your cost for running anything on the internet is never near null. The cost for an extra transaction may be, but you'll have a fixed floor.

You'll have servers to run. You'll have servers fail in a weird fashion. You'll have customers that have weird issues. You'll see GDPR requests for data. You'll have servers to patch, software to update. Wait until something comes up where you need legal guidance (ToS and GDPR compliance, for example)

You're doing invoicing for customers - they'll expect that you keep their data safe. They are legally required to keep that data for 10 years. Unless your offer is "Create the invoice from a template and ythe customer is responsbile for safeguarding their data" - in which case I can replace you with an OpenOffice Document Template, for free.