I went there same branch together with lady from agent. All what I had to show was passport and certificate of registration from Inmigration. Nobody has asked me for anything else.
Up to know I’ve got all kind of medicine without prescription. Heart tablets, blood pressure tablets, beta blocker, antibiotics. The only medicine I haven’t bought yet were psycho treatment and pain killer on drug base such as morphine.
If you don’t like to waste time at immigration, just go to an agent. For little money they will provide you with a 30 days extension. Before I had my NON-O visa I did it many times. My agent charged me 500 THB for this support. I handed over my passport, picture in the morning and afternoon same day I had my extension stamped. Don’t take the risk of overstay.