Type it, place it, print it. Exactly the size a #10 envelope actually is.
Drawn at true proportions — 9.5 × 4.125 in, same as the print.
Twelve invoices. A stack of holiday cards. Anything going somewhere that judges you by the envelope.
The alternative is a word processor template, a printer dialog you don't trust, and an envelope that comes out 94% of the size it should be — because "Fit to Page" was checked and nothing told you.
This app's whole job is that last part. The page it builds is exactly 9.5 × 4.125 inches. It is never scaled, never padded, never "adjusted to fit." Before you print, it tells you plainly which setting to pick and which one ruins it.
Being straight with you: a few printer drivers ignore custom page sizes no matter what the app sends. That's the printer, not the file — and the plain-paper test catches it in ten seconds instead of after twenty ruined envelopes.
No subscription, no ads, no account. If you want in on testing, email support@barretops.com and I'll send you the link.