Waste of my time

Ordered gas online and paid for it. I get an SMS 5 days later saying it was ready for collection. I go to the Craighall store (30 minutes later) to pick it up to only be told that they are sold out and they cannot process my order. I am annoyed because what is the purpose of online orders if they are just going to sell the item you already purchased? The manager and staff didn't seem to understand my frustration as I kept receiving the response of "but we are sold out M'am" I emailed builders directly to complain and received no response. I did receive a refund but I still feel like builders should sort this issue out. Either provide the item purchased online to those who take their time to preorder or don't provide an online service at all. Online orders should be kept separate from floor stock as the product has already been paid for by a customer!!