The fbt-discount-function Shopify Function applies the discount only when the trigger product and at least one bundle item are in the cart together.
Discount type
% off the bundle — Reduces the total cost of the trigger + bundle items together.
$ off the bundle — Fixed-amount reduction on the bundle.
% off the bundled items only — Discount applies to bundle items but not the trigger product. Common when the trigger is full-price and accessories are sweetened.
Common gotcha
If the discount doesn't apply, check that both the trigger and at least one bundle item are in the cart. The function explicitly requires both — having only the trigger or only a bundle item won't fire the discount.