Skip to content

Cart Transform Extension

The cart transform function automatically adds MUP levy charges to carts when products are priced below the minimum unit price.

What It Does

When a customer adds an alcohol product to their cart: 1. Checks if the product has alcohol units (total_units metafield) 2. Calculates price per unit 3. Compares to minimum unit price (£0.65) 4. If price per unit < minimum, adds levy charge 5. Adds levy as a child line item

How It Works

Automatic Execution

The cart transform runs automatically when: - Customer adds product to cart - Customer changes quantity - Cart is updated - Region is changed to Scotland

Levy Calculation

Formula: 

Price per unit = Product price / Alcohol units
MUP floor = Alcohol units × £0.65
Levy needed = MUP floor - Price per unit (if positive)

Example: - Product: £1.00, 2.5 units - Price per unit: £1.00 / 2.5 = £0.40 - MUP floor: 2.5 × £0.65 = £1.625 - Levy needed: £1.625 - £1.00 = £0.625 (rounded up to £0.63)

What Gets Added

When levy is needed, the cart transform adds: - Original product at its original price (with original_price attribute) - MUP Levy line item with the levy amount

Both items appear in the cart as separate line items.

Requirements

For the cart transform to work:

  1. Customer region must be set to "Scotland"
  2. Product must have custom.total_units metafield
  3. MUP enforcement must be enabled in settings
  4. Levy product must be configured and published

Customer Experience

Before levy: - Cart shows: Product × 1 @ £1.00

After levy (if needed): - Cart shows: - Product × 1 @ £1.00 - MUP Levy × 1 @ £0.63

The levy appears as a separate line item, clearly labeled.

Levy Attributes

The levy line item includes attributes: - mup: true - Identifies it as a levy - parent_line_id - Links to the original product - mup_levy_per_item - Levy amount per item - mup_total_units - Alcohol units for validation - mup_minimum_unit_price - MUP floor for validation

Troubleshooting

Levy not being added? - Check customer has selected Scotland as region - Verify product has total_units metafield - Ensure MUP enforcement is enabled - Check levy product is published

Wrong levy amount? - Verify total_units metafield is correct - Check minimum unit price setting - Review product price

Levy added twice? - This should not happen - contact support if it does - Check cart for duplicate levy items

Levy not removed when region changes? - Cart transform should remove levies when region changes - Refresh cart page if needed

For more help, see Troubleshooting.