This is really interesting because according to Reptster, you don't need to be certified. All you need to do is ship using their account. There are people using Reptster because they are NOT certified. It sounds like if you cannot ship on your own and have your own account to use in a pinch, you may be stuck and not be able to ship using the Repster account.
I did notice reading the Fed Ex terms just now that you cannot use your account to consolidate shipments from multiple people since the account is person/business and location specific. I take it that means that you cannot let a bunch of people all over the country ship using your account.