The top three
Bundle pricing (on the PDP, not just cart), progress bar to free shipping, and post-purchase upsells (thank-you page, not email).
The overrated
Cross-sell widgets on every page. They clutter UI without moving AOV meaningfully. Focus upsells at the moment of purchase intent, not browsing.
The underrated
Subscription tiers with 'Subscribe & Save 15%' on high-repeat SKUs. Recurring revenue compounds; one-time AOV doesn't.
The experiment queue
Test AOV levers one at a time. Bundle pricing first. Then free shipping threshold. Then post-purchase. Stack wins sequentially.