I think there's a chunk missing in the footage, because Zee was banging on about how he was sealing the deal at £80, but the convo he was having when Jason piped up was him saying they could bring it down to the higher cost. So either Zee doesn't know the meaning of "higher" or a lot went on, the deal dragged and Jason just seized an opportunity for time. (Although why £75 and not say £78- £79.99 is beyond me).
So Jason made one mistake, one that would've lead to him being rightfully fired if the team had lost, but there were other huge mistakes, like having no samples. That was something all three of them should've known, but no worries, the blame can be shifted to the other team for letting them go off without any.
As for the argument, it still wasn't as embarrassing as Paloma, Sandeesh and Laura on a shop step in Soho arguing about who gets the credit for selling t-shirts.