You don't need a lawyer for small claims court, but you do need proof. On the other hand, you can easily prove UPS's MO (modus operandi) in regards to InterGrad reimbursement, in which case they would have to prove you were paid for that.
It should also have occurred to you that you were cashing identical $180 checks (two of the three you received). Don't you look at the summary? It's fishy, and a good reason to question your integrity. You don't get to keep that money. And you don't get to sue for that money.
You can sue for the legitimate amount UPS owes you, whatever that may be. Even you don't seem to know. And therein lies the problem.