DS1 went off to Uni in Sept, it was DS2's 11th bday in the beginning of Oct and I packed a bday card, with a stamp on and said "he will be missing you, please dont forget to post the card". Then I texted him 3 days before DS2's bday and reminded him to post it. He didn't.
It's DS1's bday today and, of course, we all sent cards. I haven't said anything but I just think it's a bit mean that he couldn't be arsed to pop it in a postbox.
DS2 didn't remark on it so it is just me.