Power store does not have the ability to use percentages for the shipping rates, it can only use flat rates.
Also, power store shipping does not allow you to base it on the price, only on the weight.
you can fudge this by setting the item weight to be the same as the price for an item, so if an item costs 12.99 set the weight to be 12.99. This will make the weight based shipping act as if it where based on price. But the shipping rates will need to be a flat rate in power store.