Incorrect billing and unsatisfied customer

Pharmacy Direct billed me for 100 tablets on 3 occasions in 2021 but only dispatched 40 tables each time. I brought it to their attention on several occasions and advised that because medical aid did not pay anything the refund is due to me. They passed the credits on their system and reflected that they dispatched 40 tablets but refused to refund me my money. They are adamant eventhough they acknowledged that they made a mistake, that they do not owe me anything. I have told them that this is blatant ******* and a firm of *****, but they have ignored my requests. I have been dealing with several people and yet no-one is able to resolve my issue