Builders Warehouse Online Store stock incorrect

I ordered 5 Lip Channels from the Builders Online Website. Only 4 was delivered. I went in to the Strubensvalley store to ask what was going on. Firstly the lady helping me was screaming to somebody else for some reason. Or maybe it is just the way she talks? Finally she decided to speak to me and informed me that only 4 was delivered as one length was cancelled. Aparently the money for the one lenght will be paid into my account. This is the third time that I check online and see they have stock of something. Then when you arrive to collect they are very sorry as they cannot fulfill the complete order. Seem that their stock displayed is incorrect!