Enable the Knit app embed in the theme editor so the widget can mount. You do not need the theme app extension for headless endpoints or Tapcart. Those load from the Knit app origin:
Add to cart override
If a bundle has ATC override enabled, the shopper clicks the theme’s Add to Cart button and Knit adds the bundle lines instead of a single product. The widget should not show its own add-to-cart button in that mode. Developers can wrap that pipeline — see Resolver stages. If override is off, the widget shows its own add-to-cart control.Checkout
Shoppers pay in Shopify checkout. A Shopify Discount Function shipped with the app applies the bundle reward. You do not configure this per add-to-cart. Keep the Knit automatic discount Active in Shopify admin → Discounts. Do not delete it. When a shopper adds a bundle, every line in that add carries__FB_ATC_UID. The theme app extension sets this. The Discount Function uses it to group the lines, match the tier, and apply the reward. If a cart app strips line-item properties, items sit in the cart at full price.
To confirm: add a bundle from the widget, complete a test order, and check Shopify admin → Orders. Each bundle line should show __FB_ATC_UID and the discount. If the discount is missing, see Troubleshooting.