I rounded up the picnic costs and quietly kept the change

Four of us put in fifteen dollars each for a picnic. The supplies cost fifty-four. I kept the remaining six.

I did the shopping and carried the bags, and I told myself the difference covered the effort. We had never agreed to pay anyone for organising. When a friend thanked me for keeping it to fifteen each, I accepted the thanks instead of mentioning the actual total.

I've done small round-ups before. Nobody has asked for receipts or accused me of anything. Now I'm arranging the next outing, and I don't want to keep deciding privately that a little leftover belongs to me.

Should I explain the earlier amounts as well as this one, or return the six and change how I handle the next collection? I'm embarrassed that I converted a shared task into a tiny fee without asking.