You can use the Smart Cart extension if you want to process payments using PayPal/Stripe.

This extension helps adding payment links to your site. The Cart feature is also available.

You can set the Link type in the Link window. See more info about Smart Сart here: Smart Cart Features.

The Client ID is required if you use the PayPal option: How to Use PayPal Smart Cart.

If you use the Stripe option, you should get the Account key.