An in law is buying baby formula even tho I clearly stated I am breastfeeding.. What do I say?

I have clearly expressed wanting to breastfeed multiple times. And I know they know this because we only talked about it because she asked a couple times 😐 Then she sent me a message on FB telling me that I should buy a small thing of formula. I tried to gently remind her that I am breastfeeding but she said I still should in case baby doesn’t latch or I need to leave her with them. But in that case I would be pumping. She knows this, again, because she ASKED. I just didn’t respond because I was annoyed and didn’t want to start fighting. Then we saw her yesterday and she says she is buying a small can of newborn formula for when baby is with them. I told my SO that if she truly buys a thing of formula, I will take it, open the container, and dump it in the garbage so that she gets the hint since telling her clearly doesn’t do anything 😒

ETA: I don’t mind having a thing of formula, but they aren’t even asking what kind I would want to use, they’re just buying the cheapest they can find. And I know my in laws and know that they really just don’t want baby breastfeeding so they are wanting to persuade me otherwise. They don’t like that I am the only person who will be able to feed her if I BF. I also have been pregnant 3 other times with overproduction so I’m not worried !