Which printer actually works

Cheap Bluetooth printers are sold as "for iOS and Android", and a good many of them do not work with an iPhone at all. The difference is which Bluetooth is inside, and the listing will not tell you.

Why this is specifically an iPhone problem

An iPhone refuses classic Bluetooth to any device without MFi certification. A printer with only classic Bluetooth is unreachable from an iPhone — and no app can work around it. On Android the same unit works fine.

How well we know this

Verified by printing
We have actually printed a receipt on this unit.
Per the documentation
The manufacturer says so. We have not printed on it ourselves.
Reported by users
We have this from users only. Treat it with caution.
Model Width iPhone Android How well we know
Star Micronics
SM-S230i
58 mm Yes Yes Per the documentation
Bixolon
SPP-R200IIIplus
58 mm Yes Yes Per the documentation
Epson
TM-P20II
58 mm Yes Yes Per the documentation
Xprinter
XP-P323B
Sold as 58mm, but prints 80mm. It does not fit our receipt width.
80 mm Yes Yes Reported by users
Rongta
RPP02N
Classic Bluetooth only. It does not work with an iPhone at all; on Android it does.
58 mm No Yes Reported by users
Epson
TM-m30III
80 mm Yes Yes Per the documentation

The roll has to match on diameter, not just width. It is stated for every printer.