There are a few options, it really depends on the volume of sales you expect.
PayPal & Shopify Payments = 2.9% and $0.30 per transaction. Pretty basic, but if you don't expect many sales it's a cost effective option.
Authorize.net & Global Gateway (First Data) = much better pricing, for instance for Shopify users my company BizArmour does 0.4% and $0.10 per transaction. There is a $10 monthly charge which includes the gateway. As always interchange pricing is much better than the higher flat rates and there are no hidden non qualified surcharges. Here is the link if you'd like to apply for the program www.bizarmour.net/landing/Rates?source=24569.3