- It caused some upset relatives, including his mother who was really upset that her brothers and sisters weren't all invited."
I was planning a low-key wedding with only close family (e.g. those I meet and actually talk to, about 15-20 people). My mum wanted to pay for some other family to come (but only for the cost of their plate, no other contribution to the wedding was offered), but these family members were either people I had never met or people that I supposedly had met as a baby but never met again. She was upset because she is not that close to the rest of the family (she's a bit toxic and most of them have gone NC with her). My DH is almost NC with his family, so it was just mine, as we were doing a nice dinner out with his parents beforehand (who could not travel to where my family is, and my family could not travel to where he is, so it was always going to be two celebrations). It was creating so much tension just at the organising stage that we decided to scrap it all and elope. Best decision ever. We did not care about presents or cash, though. We knew most would not send anything and that was fine too. I get the impression that most of these evening invitations are saying "we did not want you there but we want a present from you.
Regarding the OP, I think it is bad form not to invite family members who you visit regularly and with whom you have discussed the wedding at length, as if they were going to be invited. OP got to hear all about the wedding ceremony and the wedding breakfast / reception, but then was not invited to it. Even if it is "his wedding, his choice", it is a rude choice. If he was planning that all along, he should not have been discussing all these details with them and specially not with OP's daughter, who has been super excited all along.
Cancel the cheque!
I mean, decline the invite!
... which you did! 