Somtimes a supplier will need a zero dollar purchase order.
Gateway will not accept a "0.00" dollar value, as it does not understand wanting to buy something that does not cost anything.
To "trick" the system, simply enter in a quantity of "1" and a price of "0.001". When added to your cart, the extended price will round to "0.00" dollars, which is what the supplier receives on the Purchase Order.
Make sure to enter in a quantity of "1", anything more than that will effect the rounding.