Price discrepancy as Builders Warehouse.

After going online and seeing their Ryobi mini tool kit with 40 piece accessories for sale for R498.00, I was in the vicinity of Milnerton and decided to pop in to their Northgate branch 28th July 2021. I collected the tool and when I got to the cashier, S. Mdunyelwa, I was told that the unit actually costs R690.00, I explained that I saw it online and that it was advertised for R498.00 and she was adamant that the unit price was indeed R690.00. I thought it possible that I could have made a mistake and paid the R690.00 and left. A bit later that day, I checked their website again and indeed it was advertised for R498.00. As I'm rarely in Milnerton, I managed to pop in at the store yesterday 31 August and enquired as to why I was overcharged for the tool. The assistant informed me that there was nothing that they could do as I purchased the unit a month ago and that I should have come in earlier to remedy the situation. Firstly, I would like to know why I paid 38% more than the advertised price and why the assistant could not have offered more clarity regarding the transaction. Unfortunately, I was running late for an appointment and had to leave without requesting more senior assistance. Could some senior Builders employee please provide more clarity regarding the above?