When you see a "+" next to the price of a product, it means the seller set a "minimum" price for his product, but you are more than welcome to give more if you think it's fair.

Simply click on the "+" or "-" buttons to add or subtract from the total. You can also enter the price manually in the price field.

You will obviously not be able to subtract more than the minimum price set by the seller, but you are free to add as much as you want!