This is an older question, but I was looking for the answer so adding here in case anyone else is looking for the answer. Some payment terminals don't require an IoT subscription, I'm using a Stripe payment terminal, and didn't end up needing an IoT box or subscription, it's a direct connection to the Odoo POS app.

One thing to note, in order to successfully connect it, you'll need to enable Developer mode or some of the fields you'll need to configure won't be displayed. I spent most of a weekend trying to figure that one out.