Question on drinking etiquette?

We went out with some of my friend’s friends. My friend’s friend, lets call her Emily, bought 8 shots (2 each) and signaled for us to come over. We took them and one girl wouldn’t drink one of hers so I asked everyone to drink it, including emily, and they all refused, so i took it. The next day I get a venmo request from Emily for 27 dollars for them (3 shots!!). Isn’t it understood that if you buy people shots and tell them to come over and take them, you are the one paying for them. When I go out with my friends I’ll buy them drinks. Now Emily is continually texting me to pay her back the money. Ill pay it back but I would like to text her a message saying something like “Usually when people buy other people shots they pick up the tab” but in more eloquent terms. Suggestions ?