I was brought up to send a thank you message when I got a present from someone - assuming I was not able to say it directly at the time. But my DILs never respond when I post them presents for their birthdays or Christmas. Part of the issue is that I then have no way of knowing if they have been delivered but tbh most of the issue is that I think it rude. My DDs will send an email or text - just with "Thanks" and that is fine. I don't expect flowing pages of gratitude! Maybe times have moved on and my ideas of what constitutes good manners is now rather out of date?