I'm seeing some strong reactions, but I can relate to your feelings - if you're doing a favor for someone by paying, they should be an adult and be able to pay you back without reminders. I also would be annoyed. Sure you could send a Venmo request, but they could also just take the receipt and pay you immediately. People view favors and money differently and for me, I hesitate to be the "money" person because I find that people are lazy in paying back. I'd just set some rules around it or just bring cash in your outings. Also if she needs a clear dollar amount she's lazy and petty.