Builders credit card payment

I am very disappointed and disgusted with their conduct. Some months they phone me to go pay my account at any of the participating shops. Some months they debit my account. Today I went and paid R1500 at Builder's only for them to debit my account for R1229. 00. In so doing they have put me in a position where I won't be able to meet my other commitments. I am furious. I would like them to tell me the exact method as to how I must pay. Debit order or cash. I would like them to reverse this debit payment. Please help me.