Incompetant voltano

For about two years i have been buying my prepaid electricity electronically ( either through cellphone banking or ATM) since i linked my account with theirs and fax the proof of payment to them and they in turn will sms the coupon. On the 13th of April 2015 i bought electricity via cell phone banking and faxed the proof of payment (the one that the bank email to me which has a reference number and all the relevant information) to them as usual and to my surprise they refused to send me the electricity. they claim that both the ATM Slip and the cell phone banking statement are not valid, that i should go inside the bank to deposit the money and fax that as proof of payment. after a heated argument between the manger and I, they finally forwarded the coupon. My problem is that I don't have the luxury of time to go inside the bank to make a deposit each time i need electricity, that is why I linked their account to make easier for me. I'm so angry. I feel they are making life difficult and its so unnecessary. Please help. I can't change them as they supply the whole complex.