There are two different types of customers. The default 'Customer' type, and the 'wholesale customer'. Wholesale customers are typically business to business customers that select all the items in their order and pay a special wholesale price not available to normal customers.
For Produce Companies
With Kiva Logic, customers can customize each order they have scheduled for deliveries. When they log in, they can view what their order contains, and make subs and shop for add-ons.
To make subs, customers click on the product they don't want, and then they are presented with substitution options based on the wholesale cost of an item. Then they click on the substitution and it will appear in their order.
If the customer changes their mind, they can always 'unsub' an item and select a different item.
Customers can browse your shop when they have a scheduled delivery. Browsing through different categories, then can add products as additional items to their order. If you want a product to be listed for sale, all you have to do is give it a retail price.
For A La Carte Companies
...screenshots coming soon.