For products that are not physically stocked but are rather provided via "Kit On Demand" functionality, the issue of syncing available inventory for a "virtual" product to your shopping cart inventory can be resolved by setting the "Virtual Inventory" attribute. You have four options to choose from on a per-product basis: None, Optimistic, Realistic and Pessimistic.
Virtual Inventory Formulas
When one or more of the components for a product built from a Bill of Material (BOM) is "shared" by multiple other products, advertising the maximum amount you could build for each product will over-state your inventory because those shared components would be represented in your overall inventory multiple times. The higher the degree of this "connectedness", the worse your overstatement of inventory becomes.
With the "Virtual Inventory" attribute you can inform the system how to apply your outlook on this conundrum. The formula you choose will be used to calculate the "Qty Advertised" for a given product and is automatically adjusted as inventory changes.
The formulas are described and compared below:
Only the currently stocked inventory for this product will be advertised. No kit-on-demand inventory will be accounted for.
The maximum available amount that can be built from the available components. Does not consider a component's "connectedness".
Depending on the most connected component in the BoM a reduced amount of inventory will be advertised.
A more aggressive formula than Realistic, the advertised amount will be considerably lower than the Realistic formula. Use this value if you are very concerned about overselling or if your inventory sync is infrequent.