Payment denied

I renewed my post box on 30 March and paid with my debit card, receiving an sms from my bank confirming payment and checking that the amount was deducted from my account. However I received a call from the post office saying the transaction did not go through and I must return and go through the process again. They do have a copy of the transaction slip so not sure what the heck is going on. I went back today after calling yesterday and they promised to call me back once this was resolved, needless to say no call received yesterday. This morning I waited around for over half an hour and had to return to work but asked that I be called and updated. I have not received a call yet and it is now 15:00 on the 1st April and have tried to call them but no-one answers the phone. What the heck can I do now as worried that my box will be locked or even given away:(