We are selling storage space on servers. Clients want to purchase more space but in fixed quantities and fixed prices.
For example, we want to sell them blocks of 100GB @ $5.00 per month, as an addon. We would like to represent this in the cart as a horizontal dialog box with a minus & plus sign on each side. They can add increments of 100GB by clicking the plus sign or removing by clicking the minus sign.
Of course we want the prices changes reflected in the cart.