Bad customer service and poor exchange policy

I purchased 4 doors from Build It and when I wanted the doors fitted, it was noticed that the doors are very big. We notified the manager, Warren, that we need an exchange and he advised that the houses in Salsoneville have smaller doot frames and they stock the standard size doors. At the time of purchase, i said that i reside in Salsoneville and no-one mentioned the different door sizes and as a customer, I did not know that interior doors and frames are in different sizes. I contacted the regional office on 23 December 2015 and Roscoe Coetzer contacted me back on 11 January 2016 as he was on leave. He and the manager both said that the doors can be exchanged but I must pay R120.00 extra per door as determined by the manufacturer. Roscoe also said that the onus to measure door frames before purchasing lies with the customer. I disagree with this. Is it not the duty of the sales staff to ask the customer if they know the size or product they need to ensure the right item is sold and then discuss additional costs? I feel that I am now being forced to pay extra per door for the exchange. I am dissatisfied with the customer service from the time of sale and not happy to pay diff now