At the optical shop

One of the plastic studs attached to the clasp on my glasses have been broken for months. Today I took it into an optical shop and asked if they could replace it. The man at the shop said they don't have an exact replacement but could change both the studs for $20. So I said okay. It took him a couple of minutes. He handed me my now repaired glasses, and I tried them on. They are perfect. I thanked him, and pulled out $20 from my wallet. He took the bills. He thanked me and said bye please come again if you have other optical needs. I said sure. I looked at him. He looked at me. Is he going to give me a receipt for that? I thought to myself. Still, he wasn't moving. He was waiting for me to leave the shop. So I left the shop.

Why didn't he offer me a receipt? Is it because he's pocketed that $20? If so, why didn't any of his colleagues say anything? Do they do this all the time? This time it's your turn to pocket the small change, next time it's me. Is $20 too much to pay for two little plastic studs? Should I have asked for a receipt? Why didn't I say anything?