MIL wants to buy her own car seat

My child is 20 months. We’ve had nothing but issues with my MIL from my birth on. In her infancy, she watched her 3 times a week until I was being constantly disrespected and she was doing things her way behind my back. Eventually, I overheard my MIL and SIL bashing me at a family event (of which they denied later), and my husband and I decided it would be best for me to be a SAHM.

It’s been almost a year now, and we have still had our share of problems...they just do not respect our boundaries.

Anyways, the last month I have tried to heal the relationship a bit and let her watch her twice a month for a few hours at her house. This last week we bought my child a new car seat (she outgrew the other one). My MIL had a base for the old car seat from when she was watching her. I have driven by her car while watching her on the road while she had my child. She never asked me if it was okay to take her anywhere. I told her I was uncomfortable with that and would like her to ask me in the future and she acted like I was being insane. I don’t trust that she would tell me in the future, a matter of fact this is not the first time she has done things behind my back.

So, having the new car seat was sort of a solution because I told her it stays in my car...it doesn’t come off the base and we will drop her off. So now she is asking us what the car seat is and where we bought it.

I DONT want her to have her own car seat. It’s totally not needed, we live 3 minutes away and she only cares for her twice a month and I don’t want her taking her anywhere. But, I also don’t want to completely offend her by telling her I just don’t trust her because I’m trying to let my child have a relationship with her grandma while maintaining my own boundaries. Ugh. What do I say!?