I have never received this offer and I use the app.
I didn't dispute what they'd said, I'm just stating that for the past year I've got 3 stamps every week using my own cup, as has everyone I know and everyone in this thread. If they are only intending to give out 2 stamps, then they need to fire their app developers.
Do the washing up and get 20 stamps....
I have always received 2 stamps whether I bring my own cup or not.
Clarification above. If you receive 3 stamps it must be an app glitch.