The Offset commerce platform is built on a "first to reserve inventory into a cart" model. Not a "first to checkout" model. "First to reserve" means once wines are added to a cart, that stock is reserved from available inventory until the customer completes their check out or the "Cart Expiration Length" reaches it's limit.
With that, if some products have a low amount of inventory available, and dozens or hundreds of customers are logging in at the same time to access their offering and load up carts for checkout, when the number of bottles reserved in pending customer carts reaches the actual number of bottles in stock, the next person that logs into their offering will see "n/a" under the purchase column, because all available inventory is currently reserved in pending carts...
For example, if you have 75 bottles of a wine remaining in actual stock, and the max allocation purchase amount for your offering is three bottles per customer, that means only 25 customers would need to load up a cart with their three bottles and at that point all available stock would be "reserved in pending carts"
Once the "reserved in pending carts" amount matches the available stock, the next person that logs in to see their allocation won't be able to add that product.
The Offset platform knows that no more bottles, at that moment, are available to be reserved into another customer cart.
A similar scenario is any physical brick and mortar store. If your local wine shop has ten bottles in stock, yet five people are walking around the store, having taken one bottle each off the shelf and put it into their shopping cart, the next customer will only be able to grab five off the shelf to put into their cart.
The store does still have ten bottles "In Stock" until people complete their shopping and pay for their bottles or decide they don't want that bottle and put it "back on the shelf" so the next person can have a chance to buy the wine.
If a customer does not complete their purchase and leaves the website, the bottle quantities in that cart are still reserved for the "Cart Expiration Length" time found on the "Settings>Carts, Allocations & Clubs" page...
We recommend setting this for 10 or 15 minutes. Anything longer can cause overflowing delays, where limited stock is being tied up in carts which have been abandoned.
This is something to consider when very limited inventory is available. If you have limited stock of a wine you should consider this scenario may occur when sending a campaign to thousands of customers.