The best place to start if you need help with a specific product is to contact the developer. All WHMCS Marketplace developers have both a website and support URL listed.
This module provides support for the Stripe payment gateway for use with WHMCS.
A complete PayPal Gateway with additional features that aren’t included in a standard PayPal Gateway!
Transfer the paypal fees to your customer
Accept Bitcoin payments and Altcoins on your WHMCS website checkout with CoinGate.com. Receive payouts in Euros, U.S. Dollars or bitcoins.
Enable "Amazon Pay and Login with Amazon" in WHMCS for single and recurring payments.