You’re not wrong to feel hurt about their not sending a thank you. Doesn’t everybody know if someone gives you a gift, you say thank you, whether it be verbal or a note like for a wedding? Heck if you read the Bride’s magazines (and I’d think everyone planning a wedding does), they tell you to thank people for gifts.
Maybe you’ll end up going to the wedding and meeting someone nice. Wait to send back the r.s.v.p. Closer to the date they asked (Did they send an invitation with an r.s.v.p. Card?)
The only invitation my cousin sent was a simple email and that wasn’t because of low budget resources. That’s how much she hates the formalities.