Custom Shipping Carrier Issue

Shopify Apps

Topic summary

A developer is attempting to build a custom shipping app that calculates rates based on order subtotal and other factors. The main obstacle is that Shopify’s DeliveryCarrierService API does not provide order subtotal information in the callback.

Key limitations identified:

  • Only line item base prices are sent (before discounts)
  • Cart-level discounts (e.g., from Cart Discount Functions) are not reflected
  • This prevents accurate subtotal calculation for shipping rates

Workarounds discussed:

  • Creating multiple shipping options for different subtotal ranges and filtering via discount customization functions (developer’s idea)
  • Using higher free shipping thresholds to approximate discounted prices (mentioned by existing app provider)

Current status: The issue remains unresolved. Discount codes can bypass shipping apps entirely, causing configured shipping rules to be ignored. This appears to be a platform limitation affecting third-party shipping rate apps broadly.

Summarized with AI on October 28. AI used: claude-sonnet-4-5-20250929.
1

Hi,
I need to implement a shipping app that will provide a custom shipping rate based on multiple factors, including order subtotal. I am trying to use DeliveryCarrierService from graphql to implement it, but the issue is that it does not send enough order info to callback uri.
To be percise, there is no order subtotal field and therefore I can’t properly calculate a shipping. The only pricing field present is lineItem price, but it provides value before line item discount(e.g. from Cart Discount Function) therefore I can’t get real subtotal.
If somebody met this issue or has any other ideas on how to implement shipping that would be really helpful.

P.S.
Right now I have only a weired idea of creating a shipping option for each possible subtotal range (12-16usd, 16-20, e.t.c.) and then filtering them in discount customization function

2

Hi @NikPaukov

Shopify doesn’t send cart subtotal to 3rd rating apps. As you mentioned, they only send product information (so base price). Most apps, like ours (Intuitive Shipping) can create a higher free shipping threshold to account for the discounted price - however, it is still limited. Depending on the discount code, it could bypass the shipping app completely, and any shipping rules that are set up, get ignored.

I hope that helps shed more light. If you have any further questions, please do let me know.

Thanks,