Legal Question in Credit and Debt Law in Pennsylvania

Sure - its not illegal per se if that is what you are asking. No seller must do business with you and if they have terms and conditions review those to see what it says if you have a balance. However, you leave out a lot of details - what kind of mail order pharmacy, why you still owe them money etc.

The solution is easy - either go to another pharmacy and fill your prescription or pay your bill and continue to do business with this company.