Most items for sale are available immediately. There are times, however, when that is not true. Items can go out of stock or very popular items, such as a new Harry Potter book, are pre-sold, that is, the book is sold before the book is even published.

When a customer adds an item to their cart that is not available, it is added to the SavedForLater area. Also, if an item in the cart, for some reason, becomes unavailable, Amazon automatically moves the item in the cart to the SavedForLater items section.

It is also possible for a customer to add an item directly to the SavedForLater items area in their cart so that they can easily purchase the item at a later date.

When items become available, you can use the CartModify operation with the Action parameter to move items from the SaveForLater section of the cart to the Active section, which is fully described in Moving Adding Items as Saved For Later. Or, if Amazon automatically moved an item in the Active area to the SaveForLater area because the item went out of stock, Amazon will move it back into the Active area automatically when it becomes available.

Many products have a limit on the quantity that can be added to a cart. Sometimes this is inherent to the product, for example, one-of-a-kind jewelry items. Sometimes it is a limit set by the vendor, for example, they might set a limit for big ticket items of one per customer. If you add an item to the cart with a quantity that exceeds the maximum, CartAdd or CartCreate automatically resets the quantity to the maximum. No error is generated. Therefore, you must verify the quantity of items in the cart with the number ordered to determine if the quantity has been capped. If so, you must notify the customer.

Digital Items, such as Kindle books and MP3 files, can be purchased only by using Amazon 1-click. They cannot be placed in the cart.

Variation parent items cannot be added to a cart. For example, you cannot add Apparel to a cart because it is not clear what apparel you really want to buy. You could add one of its children, however, such as a red shirt, size large. If you try to add a parent item, you get an error message similar to the following.

The item you specified, [ASIN], is not eligible to be added to the cart. Check the item’s availability to make sure it is available.

Variation parents do not have corresponding OfferListingId's.

Collection parent items are abstractions of the items in the collection. The collection parent cannot be added to a cart. The Collection parent is used as a name holder for the collection. Collection parents do not have corresponding OfferListingId's.

ASINs identify items, such as a Harry Potter book. ASINs do not, however, identify sales information related to those items.

As you can see in the preceding figure, an ASIN identifies an item but not an instance of it that a customer can purchase. Every ASIN can be sold by many merchants. Each merchant lists the condition and price of the item. This combination of data represents an OfferListingId. Taken together, all OfferListingIds are referred to as offers. If an ASIN does not have an OfferListingId, the item cannot be purchased.

For this reason, it is recommended that you add items to a shopping cart by their OfferListingId rather than by their ASIN.

The first time a customer wants to add an item to their shopping cart, a remote shopping cart must be created, which is accomplished using the CartCreate operation. It is not possible to create an empty cart. At least, one item must be added.

If a customer has an existing remote shopping cart, it should be used and modified accordingly. Modification can come in many forms:

The CartId and HMAC are used in the CartModify, CartAdd, or CartClear operations to modify the contents of the cart.

It is not possible to delete a remote shopping cart. Instead, it expires automatically after ninety days of disuse if there are items in the cart, or, if the cart is empty, after seven days of disuse. Carts are emptied either by the CartClear or CartModify operations, or automatically when the customer purchases the items in their cart. The expiration of the cart is reset when the cart is modified. For example, if, on day eighty-nine of disuse, a customer modifies the items in their shopping cart, the life time of the cart is reset to an additional ninety days. In that way, shopping carts can last indefinitely.

Once the items in a cart have been purchased using the PurchaseURL value, the cart is not deleted immediately but it should no longer be used. From a customer's point of view, once he or she has purchased the items in their cart and there are no items remaining in the SaveForLater section, the old cart is gone. The next time they want to purchase an item, they receive a new cart.