The Zen Cart and OSCommerce payment modules for Xoops are old. While both Zen Cart and OSCommerce are excellent as standalone versions, incorporation with Xoops is clumsy at best and the documentation is virtually non-existent. If you want to use one of these programs, the best advice I can give is to set your Zen Cart or OSCommerce system up separately and link to it from your Xoops pages.
If your site is membership or subscription driven, setting up your shop separately may not be the best option as you will have two separate databases - one for your Xoops members and one for people that purchase your products or services.
That being said, you can download and try out the Xoops payment modules for OSCommerce or Zen Cart and see if it works for you. My personal experience is that trying out both of them was frustrating and time-consuming.