There was a note when I donated that stated I was not getting anything in return for a donation, so technically, you are the one to make the donation. They are purchasing from us because we are the people willing to donate the money to different charities.
There wasn't a space for a note when I gave to them either, but when I donated for ARK (Animal Rescue Kansai), there was an area for a note, so I wrote the username and what they purchased. When I got my receipt, I copied it and emailed it to the buyer.