Baby's Ashes?

maybe you should talk to her about it

It may make sense to you that this was overstepping and I think a lot of people would agree with you; just for a moment, though, consider MIL and the possibility of her believing that she is helping you and being supportive. There's not a ton of information here, so I don't know if this is a last straw type of thing where MIL has a history of overstepping and/or ignoring your requests. If you specifically said, "Please pick up the ashes, but do not do anything with them, we already have plans," than, yup, that's not okay what she did. However, if the only request was to pick up baby's ashes, I could see how she could think it would be supportive to also get an urn and be able to give you something nice to keep her ashes in. But, you get to feel how you feel, you've had a tremendous loss. I would just be mindful when it comes to talking to MIL about it, it's already a hard time without adding hurt feelings and arguments. It might be good to talk with your partner and ask them to approach their mother and say, "We appreciate the support and the thought behind getting an urn for the baby's ashes, however, we already have something else planned." 

She thought she was doing a good thing for you. If you are unhappy about the urn, it can always be changed.

She meant well. Picking an urn for a loved ones ashes is very painful. She saved you a sad thing.

with all due respect, where else was she supposed to put them?

Would you rather have had your baby's ashes delivered to you in a cardboard box with a plastic bag inside? That's how they deliver them you know!  

maybe she was just trying to help