I never purchase stamps before so when I went to a local USPS office to get some, asked for a roll of 20 stamps... instead of asking for a booklet... anyway, I was applying for a PO box as well so I rushing and went ahead and paid for the stamps first with my credit card..later when I looked at the receipt again, I saw the $50 charge for the stamps so I asked if I could return the roll ( sealed in plastic wrap) and just get 20 stamps instead ...the lady at the counter refused to refund me because I used a credit/debit card... Is this legal?

Yes because any refund issued must be put back on your card instead of giving cash. If she refused to do any type of refund at all, you can take it up with the Postmaster. Are you sure you read the receipt right? Maybe the PO box was included and the money was shown as a total. The price of 20 stamps would be the same whether it be individual or a book. To follow up on your update, talk to the Postmaster. There is a record of all transactions. Make sure to take your receipt along.

Stamps do not come in rolls of 20. They come in rolls of 100. Twenty stamps come in books or sheets. The clerk was correct--once purchased, stamps cannot be returned.

Yup

yes, legal