Psychology beats math
$10 off an $80 order tests worse than free shipping on the same order. Even though the customer pays more in the 'free shipping' scenario sometimes.
Threshold as AOV lever
Free shipping over $X lifts AOV 15-30%. A powerful nudge that converts low-value carts into mid-value ones.
When to avoid
Ultra-low-margin products where shipping is a real chunk of margin. Then a percentage discount or tiered shipping is better.