PayPal and Square won't touch firearms at any distance — but 2A-friendly acquiring banks actively want licensed dealers. A current FFL, a compliant checkout that gates delivery through licensed transfer, and an honest card-present/online split get you placed without drama.
Card-present dealers run near-standard rates (~2.6–3.2%). Online sales with FFL-transfer fulfillment price ~3.3–4%.
FFL copy (front and back), and for online sellers: the transfer-dealer flow written out. Ammo-only sellers still need the compliance story.
PayPal, Square, Stripe — plus most "we take everyone" aggregators. It's the most policy-banned legal retail vertical in the country.