TLDR
Adyen, Braintree, PayPal, Stripe are all robust and easy to integrate. The simplest integration is so-called web drop-in. It's an all-in-one web UI via Javascript that requires minimal or no customization. The server side implementation usually comes in SDK and only requires configuration. The data flow is well controlled and funneled through merchant's server. The Javascript encrypts the secure data and passes the ciphers into onSubmit() event. There is no communication between the consumer's browser and the server.via the TL;DR App
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