If you’re using Shopify Payments in your store to accept credit card payments, you will pay credit card fees for each transaction depending on your plan (2.4% to 2.9% + 30c).
A common question merchants have is whether you will be double-billed if you’re using Shopify Payments and Paypal Express together as your payment gateways. The good news is you only pay the fees for the payment gateway that your customer uses to checkout. You will not be double-billed.
Hope it answers your question.