My SIL sent us a bill for there groceries the weekend they invited us to visit

We are all in our late 20s/ early 30s.

My SIL invited my husband and i to visit their new apartment. They live 5 hours away by train, so we planned it out as a chill little summer weekend with their other sister and ofcourse brought something small to offer them.

From Friday morning to Sunday lunch, we ate out two times (one time paid for by their father who dropped in for a visit, and the other time paid seperately). For the other meals, we helped prepare some of the dishes and helped wash the dishes too.

We had a nice time with them, but then on the train home we get a group msg saying we owe 20€ *each* (it was herself and her boyfriend, me, my husband, and their other sister) for the vegetarian groceries that weekend. We were already grumpy due to train delays but I turned to my husband and went wtf... He just shrugged and mentioned maybe the wine was pricey and that she mentioned in passing to him during the begining of the weekend when they went to do some groceries together that she would do that. His voice told me that it wasnt up for discussion - i guess essentially bc id be calling them cheap.

We have hosted them multiple times and it never crossed my mind to charge em for the meals they enjoy in our home- bc if someone is taking days off and paying their train tickets to visit us, im happy to host?! I usually just do a bit more of my normal groceries and plan maybe one *special* meal... It doesnt cost enough to ask for a contribution. If they cant afford their "special" wine grocery runs for that weekend, id have been fine with them preparing what they could afford rather than be asked to contribute.

Now they have just informed us that they'd like to visit us this October for a week to see us, and ofcourse they are welcome. I WONT ask them to contribute; its not how i was raised- the cost of hosting was never considered a financial "burden" to share with guests i invite to our home.

At the same time, im salty about our last trip. Its not necessarily the 40€ its just that our past generosity felt a bit taken advantage of. I dont like this trend but dont know how to say it politely to my husband/them.