No full refund

I placed an order on Mr Price online (order #***) and opted to get door to door delivery because I work long hours and cannot get to the mall. When my order came, the wrong size jeans were delivered. I immediately asked to return, and opted for return via courier because, as stated before, I cannot get to the mall. The return took about a week to process, which I assumed is because when Mr Price processed the returns they investigate the reason. When I got my voucher back, it was for R35 less than the value of the jeans because I opted for the courier to collect. I fail to understand why I am being charged for a mistake that Mr Price made. The person who packed the order packed the wrong size jeans so it should not be at my cost to return the jeans. When I enquired from the call centre, I was told that before returned the item I should have phoned Mr Price and they would have arranged with the nearest store for me to go in there to exchange. But where is this stated on the Mr Price site that I must phone before I return things. Also, why must it be ME who then has to go into the Mr Price store to exchange when it was the fault of an *********** Mr Price employee?!