Perhaps you really have to sit down and talk to them about the reality of the matter. If your purchase is not excessive and has a real reasonable purpose, then let them know that and ask them to prove that buying new boots after not buying anything much for ages is unreasonable.

Almost every single day of the week, save maybe Sunday, there is at least one box on our front porch from purchases my husband made. He is buying and buying and buying online all of the time. When we go to the grocery store together he always wants two carts, even though we are a two person household. He always wants to buy mass quantities of things on sale, like Kleenex, paper towels, etc. He has become obsessed with this stupid "Yes!" soup and we now have about 50 cans of it between our basement and garage. The thing is, he is 100% positively without a doubt NOT bipolar. He's like the opposite of bipolar. I am the one with bipolar disorder, and I am one of more frugal people I know. When I become manic, I do spend more than usual, but that is still usually nothing compared to my husband's regular spending. Hyperspending is a form of impulsivity in mania, but the fact is that not all manic people are hyperspenders. Maybe instead, they do other impulsive things. I do. I mean, if you don't like to spend stable, you may not like to spend that much manic. Hyperspending is not a guaranteed symptom of mania.