Builders online ordering sucks

I ordered an inverter online at the Centurion branch. It showed that 2 units were available at this store. It also indicate that you get a 10% discount if its your birth month. I completed everything and paid for the unit R9900 without getting a discount and to be collected at the branch. I saved up months to be able to purchase this. Everything went through and the order was accepted and I will be contacted. After several hours I called the call centre upon I was told that there are no stock and I have to wait between 14 and 21 days for the unit. I was unhapoy as it said that there are 2 units at this branch and I do not want to wait so long. The person then said that she emailed the branch manager which was about 15h00 yesterday 13 March. Up to now no calls or any type of correspondence. Why say you have stock and when order has been placed no stock why does the system allow you to do this of none is available. Now I have to wait the money could have earned interest and the convenience caused. I am realy very unhappy with the poor service. Im am a loyal client of theirs. Your online ordering system sucks. I need an urgent answer from them.