A product-page upsell becomes useful when it ends at a specific, available SKU that a customer can understand and add.
Start with the trigger
A rule can cover all active products or a small set of selected products. The trigger should be explicit enough to reproduce in a test store.
Choose the sellable variant
Product-level selection is not enough when size, color, or subscription options change the price. Variant/SKU selection makes the offer concrete.
Treat bundles as a defined format
Individual and Bundle are different presentation and pricing decisions. Bundle Free still depends on a specific free SKU set.
Separate preview price from checkout price
The storefront can show an estimate. Shopify Discount Functions and checkout rules determine the final amount.
A five-minute test
Pick one trigger, one variant and one discount. Save the Theme Editor block, add the offer on mobile, and compare the final checkout amount with the preview.