global cart fee Plugin used on the checkout page

-- url of the page with the problem -- : localhost
-- HikaShop version -- : 4.2.3
-- Joomla version -- : 3.9.15
-- PHP version -- : 7.4
-- Browser(s) name and version -- : All

I am working on being able to allow a customer to tick a checkbox which will then add a fixed amount to the total of the shopping cart. I have the plugin installed, and within the plugin configuaration I have set the following:

Name: Express Order
Price: +
Value: 25

Within the Cart I have added the custom field and it correctly shows on the Checkout Page.

Within Custom Fields I have created a new field with the following details:

Label: Express Order
Table: Order
Column name: express_order
Field Type: Checkbox

I have tried various different options within the value box but when on the front end I am unable to get the "Express Order £ 0.00" to increase to £25.00

Is what I am attempting to do capable with the setup or am I missing something?

Regards,
Dave

Many Thanks Nicolas

I have set everything up and it works perfectly, however I have just been told the requirement has changed, rather than have a fixed price for the service, it needs to be a percentage based on the value of the cart, exc postage and vat

Is this possible?

If not is it possible to set it up with price points based on the subtotal for example:
anything below £299.99 the charge is £35
£300 - £499.99 the charge is £45
£500 - £599.99 the charge is £75

Thanks in advance