Paypal SDK

Adaptive Payments enable developers to build apps that process transactions. App builders can control the entire transaction within a single interface. This allows payments on multiple platforms, including web, mobile, and others. Adaptive Payments make it possible to create ongoing relationships with customers so they don’t have to request user’s permission for every single transaction. Adaptive Payments offers a number of features.

With the introduction of Guest Payments feature in Adaptive Payments web flow, users can now complete a payment by simply using a credit card. This capability allows developers to highlight and market credit card acceptance and increase their conversion rate by simply redirecting users to PayPal.com.

You can create a (business) account for free with Paypal but you will pay them for each transaction.
More information: https://developer.paypal.com/