I'm imagining the next scenario. I go to shop to get a TV. Considering that I will pay with crypterium app, the price of TV is 400. I have a certain ammount of crypto that should be 400 + 0.5% CRPT of that sum. I try to pay and have insuficient funs, I have only equivalent of 380 at the moment of paying. I will tell the seller to wait a bit, because the price will increase in a few moments and will have enough funds to buy it.
Your funny story makes me think Bitcoin's value every second is moving and there is no price stability in a short time. I believe it can happen in the future and perhaps the seller will be tired of waiting for the price to rise to sell the item.