dokan_get_processing_fee filter no longer gets applied

[danielbreves]

Bug Description

In Dokan v3.14, it was possible to set a custom processing fee using the dokan_get_processing_fee filter. Since upgrading to Dokan v4, this filter no longer works as expected and the processing fee is no longer applied.
This appears to be a regression introduced in the v4 update.

Step To Reproduce

1. Add a filter for dokan_get_processing_fee in a plugin.
2. Return a custom processing fee value.
3. Place an order where a processing fee should be applied.
4. Check the commission/fee calculation.

Expected Behaviour

The value returned from the dokan_get_processing_fee filter should be applied to the vendor commission calculation as it was in v3.14.

Actual Behaviour

• The filter runs but the returned value is ignored, and no processing fee is added.
• Commission is calculated without the custom processing fee.

Additional Information

No response

Environment (please complete the following information)

1. Dokan Version: v4.2.1
2. Dokan Pro Version: N/A
3. WooCommerce Version: 10.3.6
4. Browser: N/A
5. Current Theme: Store Front