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Metafields let you drive shipping from data stored on your products rather than from rules alone. There are two distinct ways to do it, and choosing the right one matters — they behave completely differently.

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The two approaches

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Direct: the metafield holds the price

Use Product Metafield or Variant Metafield when each product carries its own shipping cost and you want that number charged as-is.
1

Create and populate the metafield

In Settings → Custom data, add a numeric metafield on Product or Variant and fill it in across your catalogue.
2

Add a rate and pick the parameter

Set Calculation parameter to Product Metafield or Variant Metafield.
3

Enter the metafield reference

  • Product Metafield — enter the Key.
  • Variant Metafield — enter it as Namespace.Key.
4

Decide whether to multiply by quantity

Tick Increase rate per item if the stored value is a per-unit price that should scale with how many the customer bought. Leave it off for a flat charge per product regardless of quantity.
5

Set a Default rate

Used whenever a product or variant has no value in the metafield.

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Always set a Default rate

If a product has no value in the metafield and no default rate is set, that product contributes nothing. The customer may see no shipping option at all.Every new product starts with an empty metafield, so the default rate is what protects you between a product going live and someone filling in its shipping cost.

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Summed: the metafield feeds a rate table

Use Sum Of Product Metafield or Sum Of Variant Metafield when the metafield holds a quantity rather than a price — a shipping score, a volume figure, a handling weight — and you want to price on tiers of that total. ShipMagic adds the metafield value across every item in the cart, then evaluates that total against a rate table exactly as it would for cart weight or item count.
1

Add a rate and pick a Sum Of parameter

Set Calculation parameter to Sum Of Product Metafield or Sum Of Variant Metafield.
2

Enter the metafield reference

Key for product metafields, Namespace.Key for variant metafields.
3

Choose a Rate Type

Range, Incremental, Range + Incremental, or Range + Range — the same four types available to weight and quantity.
4

Build the table

Fill in the bands or increments. See Ranged Rates and Incremental / Per Item Rates.

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Cover the whole scale

Because the summed value drives a table, the same rules apply as any ranged rate: bands must not overlap, and the final band should have an empty Range End so an unusually large cart still gets a price.
Products with no metafield value contribute zero. A cart made entirely of unconfigured products sums to 0 — make sure your first band starts at 0 and produces a sensible price, or such a cart may get no rate.

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Keeping the data current

Metafield values are read live at checkout, so edits in Shopify take effect immediately.
This is different from product syncing, which applies to tags, type, title, barcode, and compare-at price used in conditions. Metafield rate values need no sync.

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Examples

Each variant carries its own shipping cost (direct)
Products carry a volume, priced in bands (summed)

Ranged Rates

Building the bands a Sum Of rate needs.

Incremental rates

The other rate type available to Sum Of.

Product Attributes

Using metafields in conditions rather than as rates.

Maximum and minimum

Bounding the result.