Hello , let's directly go to the important things here , so for the payment integration there are two type 2D and 3DS , 3DS is a security way used to send sms or email to the card holder to confirm the order , anyway as i have 7 years with payment integration gateway , the work is clear ,also the documentation for SUMUP bank very details ,and this help to finish our work quickly, also i worked before with WHMCS with payment integration , let me know if you have any question how the work will go