Re: X-Payments and Incorrect CC Number
We do not use JS check in current XP version. We do "after-submit" check at PHP level.
Right now we are finalizing development of X-Payments v3.1 (release ETA end of May) where we are going to address the issue you raised here. I see you have XP v3.x license already so you will get access to this version free of charge and will be able to upgrade.
Users of hosted X-Payments plans get upgrades of their X-Payments accounts per request free of charge, too.
VP of Business Development for X-Cart