i bought a harness online from a company paid for it in full and printed off the order confirmation after telling my mate he said it was underpriced how do i stand on this i bought it in good faith from their website
You bought the goods in good faith and paid the asking price. If the goods you bought are worth more then it's not your problem as the price quoted was the price the Vendor was selling at. You are not breaking any Laws.
I would suggest you search this section as I am certain if the seller notices you have been charged the wrong price he can ask you for the excess and doe not have to sell it to you for an incorrect one
invitation to treat is an agreement between 2 parties - so if one party does not agree that invite can be withdrawn. At least that's my interpretation of that term