Hetty58
I remember being made to sit down and write thank you cards and letters. I really hated it! Therefore, I only encouraged mine to say thanks when they next saw the giver. I certainly don't expect thanks from my GC - but sometimes they'll send a text.
If you want to give presents or money - then do. If you don't want to (or can't afford to) don't. You surely don't need any excuse to stop (including the issue of not getting thank yous.)
Hetty, relatives and GPs who send moeny or presents may live a long way away- so waiting until they next see them to say thank you, is just not appropriate, imho.
It's good for children to learn that being polite and say thank you, is an important social 'skill'. I am glad my ACs have taught this to my ACs, honestly.