Passing variables FROM the store TO another portion of site

Hello!

Please forgive me if this is in the wrong place, or I'm asking a dumb question that should be self-answerable (I did search the forum):

We have an existing site (https://www.warmbloodstallionsna.com - we didn't build it, we purchased it) that is quite complex and requires specific forms to create and edit listings. There was never a shopping cart built into this site, which is part of the trouble we're running into now. What we need is for Zen Cart to allow our clients to select a product, pay for it, and then be deposited to their existing account (preferably without having to log in again) where they can either create a new listing or renew an existing one. From what I can see, we don't really need Zen Cart to have much to do with the forms themselves, we just need some way to pass the information that yes, it's indeed paid for and the client can proceed. We should only need to pass the client's existing account number (or some other identifier), the total amount (I think - may not be necessary), and the name of the listing (it's a horse breeding website where people list their stallions available for stud, so we'd need to know which stallion the listing is for).

I know there are many resources available to look at, but I was hoping someone could point me in a direction to get started, and possibly tell me that I'm not insane and that this might be possible. :) What kind of information do I need to provide so that this makes sense to someone other than me (and I'm not sure it does to me either)?

Thank you for any and all help!