Under pricing issue

Hi, I encountered a problem of pricing at Klerksoord Built It in Pretoria, where a bought a plastering sand. Everything was well and fine just waiting for delivery. Then boom, later on a cashier calls me telling me that they had under priced me therefore I need to pay the difference back or they won't deliver which I profusely denied... Then I logged a call to thier Head Office, whereby I find two ladies by the names of Mpho and Tamara who were very quick to attend to my query and within two days my sand was delivered. Keep up the good work ladies, you are the best.