Yes, this is a common sign of an inmate that is either operating a "store" or an inmate that is invloved in gambling or smuggling of contraband. Inmates use commissary items as a form of payment for services and other things that go on inside. The reason he is in the SHU is that another inmate, probably one that owes him money, has dropped a ticket on him. The COs then did a shake down but were focused mainly on his locker. If he is found to be in possession of more than the items allowed on the commissary list, he is suspected of what we outlined in the opening sentence.
The inmates know that the commissary list have a series of numbers in parenthesis next to an item for purchase that denotes how many tuna, how many mackerel, etc, an inmate may buy. If his commissary receipts show he bought ten tunas and he has one hundred, he is going to the SHU for investigation. If we are right, spending any time trying to get the BOP's attention is going to be a huge waste of time.