Boyfriends mom always yelling

Okay so we’ve been dating awhile and she is always yelling about something. My boyfriend is her oldest son and he does soo sooo much for them. Well for Christmas <a href="">Eve</a> my bf had to pick up chairs from their restaurant she never specified any specific ones so he brought the smaller ones that take less space at the table. Omg this lady practically flipped out about the chairs!!! My bf never tells her anything back but if she wanted specific chairs she should have asked. She hen snarkly says to me that I should have known to get the nicer ones. Now she’s never yelled at me but how would I ever respond. Like yes I want to be polite but she assumes everyone reads her mind!