What protoxx said. Prepaid account info is not actually considered a carrier account when required to enter carrier info. I had verizon prepaid last year and it was not recognized during checkout.
people are completely confusing SIM free and Unlocked. Unlocked means that you are not tied to a specific carrier. If you pay for a phone in full, it is unlocked.
SIM Free is essentially the verizon/sprint version of the iphone. It contains both GSM/CDMA radios.
The only phone you can order online, without carrier info, is the T-mobile verison. That is GSM only. If you pay for the phone in full it will be unlocked, but that does not make it SIM free. If you want a "SIM Fee" phone on launch you are going to have to lineup at the store and buy the verizon/sprint model and pay in full. Apple will let you do this in store.
If you are on a prepaid carrier that uses CDMA, you are out of luck unless you camp out at the apple store.