The difference lies in the type of prison and an inmate's custody level. USP is United States Penitentiary and is a high level of custody; FCI is Federal Correctional Institution which is reserved for medium and low security; and finally FPC is Federal Prison Camp which is the minimum level where there are no fences and allow work detail and liberal movement. There is no commissary "number", simply use their inmate number which would be formatted like this: xxxxx-xxx.
https://www.inmateaid.com/ask-the-inmate/what-is-the-difference-between-usp-fci-and-fpc-how-do-i-find-out-my-sons-commissary-account-number#answer
Accepted Answer Date Created: July 12,2013