If it runs in my browser, where are my invoices saved?
On your device, in your browser's local storage. They never travel to a server because Billfold does not have one. Export PDFs to keep permanent copies, and clearing your browser data will erase the saved list.
Is it really free? What's the business model?
The core tool is free forever. If you want your own logo and unlimited history, you pay $29 one time. That is the model: a fair price paid once, instead of renting access to your own paperwork every month.
How is this more private than tools that advertise privacy?
Most "privacy-minded" tools still store your data on their servers; they just promise to guard it. Billfold has no server to store anything, so there is nothing to leak, sell, or subpoena. Privacy is a property of the architecture, not a promise.
Can I use it on more than one device?
Yes, on each device independently, since data is stored locally per device. An optional end-to-end encrypted sync for people who want their invoices everywhere is on the roadmap, and even then the server would only ever see unreadable ciphertext.
Do my clients need an account to view the invoice?
No. You send them a normal PDF. Nobody signs up for anything, ever.