See, in order for their product to be available to the ZC checkout process, their product has to be added to the ZC database. Doing so on the fly (via something like an iFrame) assuming that the next time the product is displayed/purchased it is again added to the cart (not just updated) then that would introduce a slew of problems.

The easiest solution in the current arrangement of software is for their product to be provided in a format such as a csv file that can be uploaded. Any "API" that they are looking for would pretty much be from scratch or otherwise possibly commercially available.