Creative accounting - charged, and charged over and over.

Actually one star is too much. A year ago they came out , we had to pay before they would put in a new geyser. This is not a problem, but that they charged us one price, we paid, then they credited the amount and put through another invoice for less and have been emailing us constantly to pay. We send proof of payment etc, but this keeps on month after month. 'Christa Schutte' - please fix this. This is creative accounting. Can you not see theserial number is teh same on both invoices?