Since the time my kids were little I have impressed upon them that they need to write thank you's. Thanking someone over the phone is great. E-mailing is nice. Texting, much less so. Forgetting is unforgiveable. And to give me some excuse like "I don't write letters"? That's just code for poor manners.
If there is one skill you should have in your professional skill set, it's writing a letter. Businesses lose money from poor etiquette. Social and professional contacts can be lost. It's a poor reflection on your upbringing, intellect, and social skill not to say thank you for gifts received. (which is why my gift-giving list at Christmas has shrunk) Sorry, but it's true. You don't get a pass just because you think mail is antiquated.
It's not all that hard and costs you a stamp, a small notecard, an envelope. The worth of the gift justifies it. So, here's the rubric I gave to Scout when he acted like he'd never heard of writing a thank you (and by the way, that's false):
Thank you for the ________________________. I had a wonderful graduation and am looking forward to college in the Fall. I really appreciate that you thought of me!
Manners. Not really all that complicated.