Cart Binding for Add-Price and Upsell Products

Plain-language implementation guide

When added to the cart, the main product and related products include hidden properties that define their binding relationship.

The main product contains:

_scm_bind = unique binding ID

An addPrice product contains:

_scm_bind_to = the main product’s _scm_bind
_scm_inject_type = addPrice

An upsell product contains:

_scm_bind_to = the main product’s _scm_bind
_scm_inject_type = upsell

Do not hard-code addPrice or upsell detection from product titles, SKUs, handles, or product IDs. Use these hidden properties.

Implementation requirements

1. Read cart item properties

Parse the product attributes/properties/line item properties in the cart line items, and read:

_scm_bind
_scm_bind_to
_scm_inject_type
_scm_bind_qty
_scm_bind_delete

2. Establish the binding relationship

Main product: has _scm_bind.
Related product: has _scm_bind_to.
When a related product’s _scm_bind_to equals the main product’s _scm_bind, the two products are considered bound.

3. Add-on price product rules

_scm_inject_type = addPrice

Requirements:

Do not allow independent removal.
Do not allow manual quantity changes.
The quantity must follow the main product.
When the main product is removed, the bound addPrice product must also be removed.
Displayed addPrice price = addPrice unit price × current quantity.

Note: Do not rely entirely on the line_price returned by the cart API, as it may update more slowly than quantity on some platforms or themes. Calculate the displayed addPrice price using unit price × quantity first.

4. Upsell product rules

_scm_inject_type = upsell

By default:

Can be identified as a related product.
Its quantity is not required to follow the main product.
It can usually be removed independently.

5. Give rule fields priority

If rule fields are present, they take priority.

Quantity rules

_scm_bind_qty

Value	Meaning
`1 / allow`	Independent quantity changes are allowed.
`2 / not_allow`	Quantity changes are not allowed.
`3 / follow`	The quantity follows the main product.

Deletion rules

_scm_bind_delete

Value	Meaning
`1 / allow`	Independent removal is allowed.
`2 / not_allow`	Independent removal is not allowed.

6. Cart interaction requirements

When the main product quantity changes

Find all addPrice products where _scm_bind_to equals the main product’s _scm_bind.
Sync each addPrice product’s quantity to the main product’s quantity.
Then refresh the cart UI, subtotal, and total.

When the main product is removed

Find all addPrice products where _scm_bind_to equals the main product’s _scm_bind.
Remove all of those addPrice products.
Then refresh the cart UI, subtotal, and total.

7. UI requirements

addPrice products

Hide the remove button.
Hide the quantity increase/decrease controls.
Display only the current quantity.
Display the price as unit price × current quantity.

upsell products

Do not mistakenly apply addPrice’s strict binding logic unless _scm_bind_qty or _scm_bind_delete explicitly requires it.

8. Compatibility requirements

Field names may differ across platforms. For example:

properties
line_item.properties
item.properties
attributes
lineItemProperties

Please write a general-purpose reader that can extract _scm_* fields from objects, JSON strings, and property arrays.

9. Things not to do

Do not hard-code product IDs.
Do not hard-code SKUs.
Do not hard-code product titles.
Do not hard-code URL handles.
Do not merely hide buttons without synchronizing the API.
Do not update quantity without updating the displayed price.
Do not change only the frontend while leaving addPrice items after the main product is deleted.

Final goal

Users operate only the main product.
addPrice automatically follows its quantity and deletion state.
Upsells are recognized without being incorrectly forced into strict binding.
When new addPrice or upsell products are added, no theme-code change is needed as long as their cart properties are correct.

Developer quick summary

In one sentence:

In the cart, implement related-product binding using _scm_bind, _scm_bind_to, and _scm_inject_type. The addPrice product must follow the main product’s quantity.