Have they cashed the postal order?
If they have been asking you to make a payment because you owe them money, they can accept this as part payment and reject the offer of final settlement.
You should have written offering the amount as full and final payment, and they would have replied either accepting or rejecting it. If they had accepted they would have attached a condition that the payment reach them within a certain date.
When you made the contract with them you accepted their terms. Now you want to change them, you have to wait for their agreement.