Besides being good etiquette, a TY note lets the giver know that the intended recipient received the present. Packages and envelopes can be stolen, misdelivered, lost in transit, etc etc etc. Donations to online funds can go astray. I give the couple a few months and then if there’s been no TY I’ll ping them (or their parents if that’s the only contact info I have - hate to play that card but so it goes) to ask if the present I sent went missing.