Salesforce B2B Commerce has some limitations in that it does not allow to directly edit or copy templates to override some of the content of modules like Mini Cart, Product Details page, etc. To override these modules, template code needs to be taken from the source code and the modification needs to be done there.
Stripe is a Payment gateway plus a merchant processor. You can take credit cards directly from your websites. Stripe is free but there is fee for the transaction. Stripe account can be created using the following link https://stripe.com.