How to handle thank you notes

tealeavesandpeonies

So, the ILs decided to throw us a shower last weekend.  They live three hours or so North of us.  The night before the shower we ended up in L&D until 5 AM and when we woke up Saturday morning (day of the shower) DH had a bad fever and felt too sick to go.  We called the hosts to let them know that due to the extenuating circumstances we would not be able to make it.  

MIL decided to go ahead and have the shower without us and then proceeded to open all of our gifts.  She posted pictures of some of the gifts on Facebook,  but as she invited all people that I've never met before, I have no idea who these people even are nor who got us what.  

How should I go about getting thank you cards out to these people?  We will not be seeing ILs until after the baby is born due to a history of bad behavior on their part and subsequent temper tantrums from MIL over the past week, thus we will not have the gifts until then, if at all.  

I don't want to offend the party guests though.