PayPal is one of the most popular global payment provider. Accept credit cards and PayPal payments including PayPal Credit® (formerly Bill Me Later®) online with PayPal Payments Express Checkout. This all-in-one solution offers an embedded checkout that keeps customers on the merchant's site and an Internet merchant account from PayPal. Plus, it's PCI compliant to help merchants manage their credit card security requirements.
1. Customer enters your store and add products to cart.
2. Login to his PayPal account or enter credit card details.
3. Confirm the purchase.
4. That's all.
More info you can find: https://www.paypal.com/us/webapps/mpp/home