oomis,
There are plenty of inexpensive TY notes or buy a pack of pretty paper(100 to the pack) and make your own. It does not have to cost much.
It really is NOT enough to say an obligatory thank you at the wedding especially when you have no idea what the gift is, until you open it.
Its not too much to ask them to show that they are makir tov.
I agree 100% with these points. The question is, however, if the giver did not receive a thank you card, should s/he dan l’kaf zechus/be mochel the recipient? Or should s/he consider it pure rudeness or unappreciative?
My point was that I don’t think the most likely reason in the majority of cases is rudeness or unappreciative feelings, but rather a lack of organization or carelessness. These are also faults, but perhaps less infuriating to the giver than straight up lack of appreciation.