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WooCommerce Flat Rate Shipping Formula Causes Validation Error

open Sep 22, 2025 Plugin → Woocommerce

Summary:

A merchant is unable to use a specific shipping cost formula in their WooCommerce Flat Rate shipping settings. The formula [fee percent="10" min_fee="10" max_fee="500"] triggers a validation error stating "Please enter a valid number" when entered, preventing the merchant from saving the shipping method configuration.

The issue is specific to the merchant's environment, as the reporter could not replicate it on their own test site. The problem persists even when the staging site is placed in conflict testing mode, suggesting it may not be a simple plugin or theme conflict. The WooCommerce version on the affected development site is up to date.

This bug directly impacts the merchant's ability to set up flexible shipping rates based on a percentage of the cart total, which is a core e-commerce functionality.

How to Replicate:

  1. Log into the provided development site.
  2. Navigate to the WooCommerce Flat Rate shipping settings.
  3. Enter the formula [fee percent="10" min_fee="10" max_fee="500"] into the cost field.
  4. Attempt to save the settings.
  5. Observe the "Please enter a valid number" error message.