Invoices, with PDFs.

One business per account: clients, numbered invoices with line items and a single tax rate, a PDF for every invoice, a shareable link, and payments recorded by hand. Sign-in is the door's — this app keeps no password and no session of its own.

What it does not do: compute tax for a jurisdiction, send email, or take online payments. Which invoices it is fit to produce is written down in TAX.md.