It is bad manners not to say thank you, but I think these days one does need to accept that handwritten notes are going the way of the dodo.
No excuse not to have sent a text, though, if it's been some time. I was always taught you had 6 weeks, following wedding or baby.
I have many relatives on the other side of the world who would always send a gift for a wedding if we sent invitations. They would send baby gifts if sent a proper birth notice. My mother certainly expected us to write appropriate thank you letters, which was even harder since it had to be in their language, which was very much a second language to me, not my home language.
These days, if my MIL sends something for DS (7), I send a quick text saying it arrived safely, then DS sends an ecard to say thank you. He enjoys selecting nice ecards.