Of course if you mom uses your stepdad's credit/debit card to buy groceries, he wants to see the receipts, in order to know what amount to deduct from his checking account, or how much is going to be charged to his credit card. If your mom has her own business, then she needs to get her own card, and stop using his.
It sounds as if your stepdad has his own private checking account, but your mom deposits her money into an account that is set up for the specific purpose of paying for joint household expenses. That is an arrangement that they agreed upon between the two of them. She can always open up her own private checking account and deposit some of her money into her own private account, if she wants to do so.
If your stepdad wants to spend every cent he earns on wine and cheese, that is his business - as long as he is contributing to the household expenses. Which he probably is doing, but they have seen no reason to involve you in their personal financial arrangements.
Of course your stepdad is always buying things for his biological son. He's SUPPOSED to. It's not his responsibility to buy things for you or your siblings. That is your mother's responsibility - not his. He sticks his nose into it and wants to know "how much it cost" because your mom is using HIS credit card to pay for it.
Keep your own nose out of their financial affairs, of which you know nothing about.