Dear Pizza Delivery Guy,
When you told me that you'd forgotten my Diet Coke and were going back for it, I felt bad that you would have to make an extra trip and possibly fall behind on your other deliveries. I felt so bad that I got out a couple of dollars in cash so I could give you an additional tip on top of what I'd already put on my debit card.
But you've been gone for almost an hour, Pizza Delivery Guy. I know where the pizza place is, and it isn't that far away. I'm finished eating and I want my Diet Coke, and my sympathy for you is diminishing with every minute in which you fail to return.
Get a move on.
Your thirsty customer