Hi all,
Version 7.8 has now reached public release candidate, which includes this feature for the Stripe payment gateway: https://preview.whmcs.com
A new option called "Allow Payment Request Buttons" has been added to Setup > Payments Payment Gateways controlling this feature. The Android Pay, Apple Pay or Microsoft Pay button will be displayed on checkout depending upon the visitor's browser.

Please join us during the pre-release testing period and provide your feedback in our dedicated beta community board: https://whmcs.community/forum/471-v78-beta-discussion/