When you enable 3rd Party Shipping rates, your customers will see live shipping rates from your integrated shippers at checkout. They can choose their preferred carrier and rate. This allows you to charge the actual shipping cost instead of using weight or price-based rules.
Note: This feature requires merchants to have the "3rd party carrier calculated shipping" available on their shopify plan.
Refer to: 3rd party carrier calculated shipping
Note: If you activate the 3rd Party Shipping Calculator, delete any Shipping profiles in your Shopify settings. Otherwise, both the app-created and Shopify profiles will appear at checkout.
Steps:
Before proceeding, integrate at least one shipping carrier and ensure all vendors have associated warehouses. Accurate shipping rates require each vendor to have a warehouse set up. If any vendor lacks a warehouse, backup rates will apply to the entire cart.
For carts with products from multiple vendors, the total shipping rate will be the combined sum for all products.
Note: Shopify limits the time allowed for calculating shipping rates. Since shipper APIs can take from less than a second to several seconds, if a shipper's API exceeds the allowed time, Shopify will apply a backup rate. Shipping rates can be calculated for up to four shippers, so deactivate any shippers you don't frequently use to prevent issues.
If no default dimensions are set for your cart, the rates will be calculated using incorrect dimensions (1x1x1). Set dimensions at the product level and add a default dimension under Settings -> Shipping Partners -> Shipping Label Settings -> Create Default Package Dimensions For Shipping.
Setting Up Back-Up rates on Shopify
In rare cases where the rate is not applied or takes more than 10-15 seconds, Shopify will use backup rates. When this happens, Shipturtle will send an email to the merchant.
Refer to: Setting Up Back-Up rates on Shopify