In the 1930s, a behaviorist named Clark Hull watched rats run mazes and noticed something. The rats ran faster the closer they got to the food. They were not steady the whole way.

Psychologists call it the 'goal gradient' effect, and humans do the same thing. We accelerate as we near a finish line. It's why loyalty cards with two pre-stamped squares get completed more often than blank ones with the same number of stamps to go. The visible nearness of the goal pulls you forward.

Most e-commerce stores' free shipping threshold runs on exactly this logic.

When a customer has $47 in their cart and free shipping kicks in at $50, they don't feel $3 away from spending more. They feel $3 away from winning. So they add the item. 58% of shoppers do exactly this (SellersCommerce research).

Which means the threshold number isn't a shipping setting. It's a finish line you're drawing for your customers. And most stores draw it once at launch and never move it again.

Here's the trap. Your AOV drifts up over time as you add products and raise prices. But the threshold stays frozen. Eventually customers are already past the finish line before they start. No pull. No goal gradient. You're just absorbing shipping costs for nothing.

One store we looked at: $50 threshold, $62 AOV. Customers cleared it without trying. They moved it to $65 and recovered $40K a month. The fix was easy and worth it (how many of those do you get!).

The rule of thumb: set the line 15 to 25% above your current AOV. Close enough to chase, far enough to matter. And check it every quarter!