I am very confused by your post. You should never have to pay for a software repository unless it is some sort of propiertary software but that wouldnt make sense anyways because once they give you access to the packages you could easy download them and share them throughout the web.
Here is a general article about how to create repo's using an ISO:
http://www.itzgeek.com/how-tos/linux...#axzz3FUfDq2MV
It sounds like you may have been scammed in some fashion, I was curious and tried to google for a paid software repo and wasnt able to find anything close to legitimate.
If you don't mind would you share a bit more about your story and the vendor so we can get more perspective on what you are trying to accomplish if its not just mounting a Linux Mint ISO and using it as a repo?