The Shopify POS app, on iOS, has a receipt printer configured. It prints an order detail automatically. I developed an embedded app, generating a pdf invoice. When I try to print, I can't detect the printer, because it isn't AirPrint compatible.
How can I programatically send the invoice to the receipt printer?
That's a tough one, only AirPrint compatible printers can be used on iOS natively. However, it may be worth looking into Google Print. I beleive it allows you to setup a "virtual printer" that then pushes it to your printer that way. Either that, or your app could send an email with the print job (pdf) to the printer, if it supports that.