Fees per Pouch, per Gram or per Milliliter on Shopify: Using Metafields

Last updated 23 September 2026

Some fees are defined per unit of something Shopify does not track: per pouch in a can, per gram of sugar in a drink, per gram of plastic in a package, per milliliter of liquid. Shopify has fields for price, cost and weight, but not for these. The way to charge them is to store the number in a product or variant metafield and let the fee multiply it: fee = rate × metafield value × quantity. In FlexiFees that is the Per unit of metafield fee type, and Percentage of metafield does the same for a percentage of a stored amount, such as a wholesale price.

What kinds of fees need a metafield?

Any rule of the form “X per unit of Y”, where Y is a property of the product:

The amounts in any real rule come from whoever sets it: a tax agency, a producer-responsibility scheme, or your own pricing. FlexiFees performs the calculation you configure; check the rules that apply to you with your advisor.

How do I set up a fee per pouch?

1. Create the metafield definition. In your Shopify admin, go to Settings > Metafields and metaobjects, choose Variants, and add a definition with the Integer or Decimal type, for example custom.pouches_per_can. In the definition, select Storefronts so the value can be read in the cart.
2. Fill in the values. Enter the number on each variant, for example 20 on a 20-pouch can and 15 on a 15-pouch can.
3. Create the fee. In FlexiFees, choose Per product fee, set the fee type to Per unit of metafield, and enter the rate per unit, for example 0.05.
4. Pick the metafield. Under Metafield that supplies the number, choose Variant metafield and pick custom.pouches_per_can from the list of your store’s definitions.
5. Choose the products with a collection, type, tag or product category condition, and publish.

Result: three 20-pouch cans pay 0.05 × 20 × 3 = 3.00, as one fee line.

Product or variant metafield?

Use a variant metafield when the value differs between variants of the same product: a 10-pack and a 20-pack, a 330 ml and a 500 ml bottle. Use a product metafield when every variant shares the value. Both are available in the fee types and in formulas.

How do I charge a percentage of a wholesale price?

If the price the rule refers to is your Cost per item in Shopify, the existing Percentage by cost fee type already does it. If it is a different figure, store it in a money metafield such as custom.wholesale_price and use Percentage of metafield: 40% of a 2.00 wholesale price on three units is 2.40. See Percentage of metafield.

What if the rule has more than one part?

A Formula fee can combine a metafield with anything else:

• A per-pouch rate plus a percentage of the price: 0.05 * mf("custom.pouches_per_can") * qty + 0.10 * price * qty
• A percentage of a stored wholesale price plus a fixed amount per unit: 0.40 * pmf("custom.wholesale_price") * qty + 0.65 * qty
• A cart-wide total from a variant metafield: 0.05 * sum_mf("custom.pouches_per_can")

In the editor, Try it on a real product shows the value each mf(...) or pmf(...) finds for a product from your store, which is the quickest way to spot a missing value before you publish. The formula reference lists every variable.

Why is my metafield fee not showing?

• The value is empty or text. Metafield fees need a number, a money amount or a unit value such as weight; anything else counts as 0, and a fee whose total is 0 is not added.
• The key is spelled differently. Pick the key from the definition list instead of typing it. FlexiFees shows a quiet warning when a typed key matches no definition in your store.
• Storefront access is off on the definition, so the value cannot be read in the cart.
• It is the first cart with a new amount. Shopify can take a few seconds, occasionally up to about a minute, to accept a newly calculated amount; FlexiFees retries automatically.

More in Metafields in FlexiFees. For a complete example with four calculation methods side by side, see the nicotine and vape fee guide.

Sources

• Shopify Help Center, Creating custom metafield definitions
• Shopify Help Center, Adding values to metafields