What’s the best for no monthly fees, low start up?
If you are using Quickbooks, they have a processing system with no monthly fee.
Almost all card processors will play the “shell game” with rates and fees. They promise one thing, and then add mysterious fees and surcharges after the fact.
If you want total transparency, try PayPal Here or Square (or similar) which are flat fee with no surprises, and no monthly fee if you don’t process any cards.