Tracking sales + tier upgrades

Sales are tracked through partner links.

1. Send each partner a link to your store ending in ?partner= and their code, for example https://yourstore.com/?partner=BLOG10. Any page works, such as a product page.
2. A shopper who opens the link is remembered for 30 days in that browser. Their next order is linked to the partner (first partner wins; an order can't be moved to another partner).
3. When that order is completed, the sale is added to the partner's Sales History with the order total, and the commission is worked out from the partner's tier rate at that moment.
4. Check Tier Upgrade runs after each sale and moves the partner up when their total sales reach the next tier's target.

Only active partners earn. Orders cancelled before completion earn nothing.