add custom field price to order

Hello,

The "custom field price" plugin is there to add extra prices to an order while the customer is in the checkout process.
Editing the order afterwards won't allow to affect the final price using that plugin.

More than that, a vendor do have access to a "sub order" and not to the customer order, which means that a vendor cannot modify the sub order (or the customer order).
The special mode to avoid the usage of "sub orders" is possible but it will affect highly the way how the vendor can act since you place the rule that only one vendor can be assigned to an order and that the vendor becomes the "owner" of that order.
That kind of configuration is not really recommended if you have fees on the vendor sales.

Regards,

Hello,

For such kind of fee on a delivery, the best is to use the fee on shipping methods.

Regards,