I'm with your DH on this.
It's not petty, it's rude to consistently forget how to spell someone's name correctly, IMO.
My name is one that can be spelt one of 2 ways, with or without an extra letter on the end. Mine doesn't have it. Lots of people add the extra letter. One relative, who has known me ALL of my 50+ years, has ALWAYS spelt it with the extra letter. This makes me really cross!
When I get emails sent to me with the extra letter added, I send them back to the person, adding the same extra letter to the end of their name. This has worked for me in about 2/3 of cases, but it is a bit passive-aggressive and I know that. I find emails even more annoying because my name is right there in front of them, on the email address - they have no excuse apart from "oh I always spell it like that". Well, that's not my name.
So no, neither you nor your DH are being unreasonable about this - it's really NOT hard to get a single letter issue sorted in a name, and it's bloody rude not to, especially when you've been told about it!
I do like the idea of sending them cards with the last letter of their name missed off too though. 