Actually DP and I aren't married yet but for convenience sake I'm calling her MIL.
It came about because she gave me a card and book for my birthday. Obviously I thanked her, gave her a hug and kiss, thanked her again saying goodbye and emailed FIL the same evening to say thank you (MIL doesn't use Internet or have email address).
This was a week ago. This afternoon I received a an email from FIL saying MIL was upset she hadn't received a card yet but that he was sure it was in the post.
Truth is I did forget as its been frantically busy at work and initially when I read FIL's email I was mortified. But now I'm a bit annoyed and don't want to send one at all because it seems to me a complete overreaction although I realise I'm probably being a bit petty.
This isn't the first time and it's probably my fault for making a rod for my own back as I've always send cards after it arose the first Christmas I met PILs and I received a lengthy letter from MIL how rude and lacking in manners she thought it was, and that she was surprised as I had seemed "such a nice young lady". I should have just shrugged it off in hindsight but being a bit shy and lacking in confidence back then, I was embarrassed and felt terrible that I'd upset her.
Now though, her drama lama ways just irritate me.
My family don't expect thank you cards. And I was brought up thinking a call, text or email is fine.
DP says he gets stick too for not sending cards and that I should just ignore it because she'll only find something else to moan about. 
I'm about to go to the supermarket now and can easily pick up a card.
So, to card or not to card?