My main question is that since cannabis is still prohibited federally, would be illegal for cannabis sales data and payment processing to cross state lines?
No. As it is an interstate transaction, the applicable laws of sales are the Federal Laws, which still classify it as an illegal substance, and thus, will prosecute for charges related to interstate sales. In so far as I am aware, all cannabis grown for consumption (as opposed to other utilities, like Hemp rope and oils, which I'm fuzzy on the legal specifics) must be grown and sold within the same state to avoid Federal issues.
That said, if a non-Colorado resident was vacationing in Colorado, they could purchase cannabis for consumption while in Colorado and provided they keep it in the state, are allowed to use it... However, they may not take it home with them (This again becomes Interstate or International and the Feds have jurisdiction over that.).