Been there with my mother. Remember as a teenager saving for ages for a Benetton white cotton sweater. Bought it and proudly spread it on my (perfectly made) bed to admire, then went out.
When I came back it was missing and mummy dearest was gone to bed. Eventually found it "carefully hand washed" and hung on the clothes line by the bottom seams, not even the shoulders, while soaking wet.
Totally beyond reshaping.
But it was "my fault" as I "shouldn't have left clothes lying on the bed" or she wouldn't have thought it needed washing.
I feel for you op, these bitches get some sort of warped pleasure out of ruining things then acting like they were doing you a favour. Then they can whinge to other people about how ungrateful you are.
It's obvious from this thread that there are a lot of lucky people who can't imagine this being done to them. I have to admit being jealous. It must be lovely not to have this passive aggressive shit going on.
Don't send her an invoice, this is exactly what she wants. She will whine to anyone who will listen to her about what an ungrateful DIL she has and will wave your invoice in their faces as proof.
If you must stay in MIL's again, I'd suggest:
a) Get a bag with a lock on it to keep your stuff in.
b) Send her a message thanking her for her help and explain that your clothes are carefully selected and require very careful handwashing, and that her loving gesture has cost you a lot of money, but never mind, it's so sweet, every time you think of your ruined "whatever", you will think fondly of her misplaced efforts to help. "Such a cute thing for you to do even though it didn't work out, luckily DH's clothes escaped the same fate ha-ha-ha".
So instead of her painting herself as someone who was only trying to help but didn't get it right, you're doing that and saying you forgive her (even though you may not be forgiving her). This is not what she wants.
She will probably make some snide reference next time you stay like "I better not wash that in case you think I'd ruin it". Your response is a tinkly laugh "Do you remember MIL when you destroyed so much of my clothes when you were kindly washing them (even though they were clean). I think it was the sweetest gesture gone wrong ever. Does anyone else have a story to top this one?" Go on, and on, and on about it, it was SOOOO funny, and the SIZE of the whatever compared to how it used to be, and you meant SO well you must have been GUTTED when you realised what you'd done, etc. etc.
She will know exactly what you mean and will shut the fuck up. (IME).
A possible c) is that you "return the favour" - grab every stitch of hers you can put your eyes on. Shame about that red sock that someone left in the washing machine ...