Respectfully, when you give someone money as a gift, what they are spending it is not really your concern. You are gifting it to them, and they are free to do with it as they wish. Out of curiosity, what is something that would upset you if you found out they used the money you gifted them to make the purchase? I'd agree with PP that saying what you're putting the money towards in the thank you notes is a nice way to let them know, if you're so inclined. It does make it more personal. One thing I roll my eyes at are the pre-printed thank you cards that people are sending more frequently nowadays. I get that they are sending a ton of thank you's, but its nice to see a handwritten or personalized one come through so you know they took the time to consider your gift and write out a message to you.