Amazon's $79 a year made shipping a subscription.

Before Prime, Amazon charged a fee per order and required a minimum for free shipping, so a shopper at checkout was faced with the cost and the wait, and this close to the sale was where carts were abandoned.

In February 2005 Amazon launched Prime at $79 a year for unlimited two-day shipping and overnight shipping for a small extra fee, with no minimum order, so the buyer no longer paid per shipment.

Removing the shipping fee from the decision meant members shopped more often and across more categories, and Amazon used the membership fee to fund the faster, cheaper delivery the whole network would grow to need.

Metre the shipping fee into a subscription

A flat annual fee removes a variable cost from every single checkout.

Members order more often, so the fixed membership cost is spread over more sales.

The fee funds delivery promises that pull more catalogue and more customers in.

Once a shopper has paid, the shipping is sunk, so the decision to add items is easy

Charge for the service once and remove it from the checkout, and the barrier to each purchase disappears.

Prime became one of the most valuable memberships anywhere and grew from shipping to video, music and more, and it pushed e-commerce toward free, fast delivery as the baseline. Amazon kept the price flat for years despite rising shipping volume.