You're still not getting it.
We "pay a bill" when someone provides goods or services in advance and we pay after receiving them. When we pay an electric bill we are paying for the electricity we used last month. When we pay a restaurant bill, we are paying for food we have already eaten.
At a convenience store (and most other retail stores), we gather things ourselves from shelves and we take them to a counter where a worker rings them up. Then we pay. As GoesStation pointed out, your situation happened in the past, so past-tense verbs should be used:
When I went to pay at the convenience store, I couldn't find my wallet.