Each supplier has a minimum order amount, which must first be met in order to submit an order. The dollar value of your order must total at least this outlined supplier minimum, otherwise you will not be able to checkout your cart.
Minimum order amounts vary based on supplier.
Once you have added an item to your cart, the supplier's minimum will always be displayed at the top of your cart.
If you add items from multiple suppliers, separate minimum order amounts will be displayed and must therefore be met in order to checkout your cart.