Honestly, just use their respective APIs: you can get far with just Stripe Checkout (https://stripe.com/checkout) and Paypal Checkout (https://developer.paypal.com/demo/checkout/) and they are both simple enough. Paypal Checkout even has an entirely client-side payment flow.

The fact that Paypal keeps most of their fee if you have to issue a refund does suck, but that is a separate issue: the interfaces are both very straightforward.