There’s two options, depending on your ite.
First, check with wahtever e-commerce pluign you’re using, and find thier implementation of it. There’s a lot around, but it will dpeend on wich one you’re using.
Second, if there’s nothing available for your system, use the ZPI documents that they provide to set up your sit to call their system and deal with their callbacks.
I have done this (not in WordPress, but in a different propriatary system) and it’s not really that hard if you have a bit of an idea of what you’re doing. If you’re not a programmer, then I’d suggest that you hire someone to do it for you as it’s not a small job, and you really will need to get it right in order for the bank ot sign off on yor system.