I did this before and accidentally clicked money order. They can not change what you've already selected so you HAVE to go to the bank and write a money order in order for them to release your application. There's no use trying to change the option because trust me, I've tried. I've email PharmCAS and explained the situation and their answer was a strict NO. The longer you stall, the longer it will take for them to release your application to the schools so I'd suggest you go to the bank tomorrow and mail it out as soon as possible.