Custom Shopify Volume Discount Function Developer

I'd build a Cart Transform Function that reads bulk pricing tiers stored on each product's metafield and applies the right discount instantly at checkout. On the storefront, I'd add a tier preview widget so customers see their savings threshold before they buy. The whole thing lives in product metadata — no codes, no manual intervention.

A mid-market DTC merchant sells products where bulk purchases are common and wanted a way to apply tiered discounts based on quantity without requiring customers to enter a code. The merchant needed the discount to calculate automatically at checkout and display the savings tiers to customers in real time as they adjusted quantities.

Four pieces

Cart & Checkout

Bulk Tier Reader

Reads the bulk pricing tier data you've stored on each product and matches it against what's in the cart right now.

Shopify Function + metafield

extensions/cart-transform/src/run.graphql function

query Input {
  cart {
    lines {
      id
      quantity
      merchandise {
        __typename
        ... on ProductVariant {
          id
          title
          product {
            id
            title
            metafield(namespace: "bulk_pricing", key: "tiers") {
              value
            }
          }
        }
      }
    }
  }
}

Store tiers as JSON in the metafield: [{"minQty": 5, "discountPercent": 10}, {"minQty": 10, "discountPercent": 15}]

Cart & Checkout

Tier Discount Logic

Calculates which tier each line qualifies for and computes the exact discount amount per item.

Function script

Storefront

Real-Time Cart Preview

Shows customers the exact savings they'll get at each tier as they change the quantity in the cart or on product pages.

Theme extension + cart drawer

theme/snippets/bulk-tiers-preview.liquid liquid

{% if product.metafields.bulk_pricing.tiers %}
  {% assign tiers = product.metafields.bulk_pricing.tiers | parse_json %}
  <div class="bulk-tiers-widget">
    <p class="rte"><strong>Buy more, save more:</strong></p>
    <ul style="list-style: none; padding: 0;">
      {% for tier in tiers %}
        <li style="display: flex; justify-content: space-between; padding: 8px 0; border-bottom: 1px solid #eee;">
          <span>{{ tier.minQty }}+ items</span>
          <span style="font-weight: bold; color: #27ae60;">{{ tier.discountPercent }}% off</span>
        </li>
      {% endfor %}
    </ul>
  </div>
{% endif %}

Render this snippet on the product page and in the cart drawer; update quantity input to re-render and show which tier the customer is about to hit.

Operations

Metafield Setup & Admin Dashboard

A simple form where you set bulk tiers for each product — the Function reads it automatically.

Custom admin app

Admin GraphQL explorer graphql

# Admin GraphQL — set bulk pricing tiers on a product
mutation SetBulkTiers($ownerId: ID!, $tiers: String!) {
  metafieldsSet(metafields: [
    {
      ownerId: $ownerId
      namespace: "bulk_pricing"
      key: "tiers"
      type: "json"
      value: $tiers
    }
  ]) {
    metafields {
      id
      namespace
      key
      value
    }
    userErrors {
      field
      message
    }
  }
}

# Variables:
# {
#   "ownerId": "gid://shopify/Product/123456789",
#   "tiers": "[{\"minQty\": 3, \"discountPercent\": 8}, {\"minQty\": 10, \"discountPercent\": 15}]"
# }

Build a simple React form in your admin app that collects tiers and calls this mutation for each product you want to enable bulk pricing on.

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