suppliers, or have several but not all of these indicators, revenue is recorded gross. If we are not primarily obligated and amounts earned are determined using a percentage, a fixed-payment schedule, or a combination of the two, we generally record the net amounts as commissions earned. 

Product sales and shipping revenues, net of promotional discounts, rebates, and return allowances, are recorded when the products are shipped and title passes to customers. Retail items sold to customers are made pursuant to sales contracts that generally provide for transfer of both title and risk of loss upon our delivery to the carrier. Return allowances, which reduce product revenue by our best estimate of expected product returns, are estimated using historical experience. Revenue from product sales and services rendered is recorded net of sales taxes. Amounts paid in advance for subscription services, including amounts received for Amazon Prime and other membership programs, are deferred and recognized as revenue over the subscription term. For our products with multiple elements, where a standalone value for each element cannot be established, we recognize the revenue and related cost over the estimated economic life of the product. 

We periodically provide incentive offers to our customers to encourage purchases. Such offers include current discount offers, such as percentage discounts off current purchases, inducement offers, such as offers for future discounts subject to a minimum current purchase, and other similar offers. Current discount offers, when accepted by our customers, are treated as a reduction to the purchase price of the related transaction, while inducement offers, when accepted by our customers, are treated as a reduction to purchase price based on estimated future redemption rates. Redemption rates are estimated using our historical experience for similar inducement offers. Current discount offers and inducement offers are classified as an offsetting amount in “Net sales.” 

Commissions and per-unit fees received from sellers and similar amounts earned through other seller sites are recognized when the item is sold by the seller and our collectability is reasonably assured. When we are responsible for fulfillment-related services, commissions are recognized when risk of loss and title transfer to the customer. We record an allowance for estimated refunds on such commissions using historical experience. 

Inventories 

Inventories, consisting of products available for sale, are accounted for using the first-in first-out (“FIFO”) method, and are valued at the lower of cost or market value. This valuation requires us to make judgments, based on currently-available information, about the likely method of disposition, such as through sales to individual customers, returns to product vendors, or liquidations, and expected recoverable values of each disposition category. Based on this evaluation, we adjust the carrying amount of our inventories to lower of cost or market value. 

We provide fulfillment-related services in connection with certain of our agreements. In those arrangements, as well as other product sales by other sellers,