You've certainly used (or know of) a product or service where there was a "Buy" button that was actually Wyre's whitelabel "fiat onramp" service -- essentially a widget they add to their website, or a couple API calls from an app.

Mostly used for ACH (US) and SEPA (EU).

The wallets add this and earn a commission on each sale that occurs.

Others in the "fiat onramp" API space are::
MoonPay, Ramp Network, SimplexCC, Transak, Mercuryo, OnRamper, Banxa, and a handful of other smaller ones.