With existing Odoo features you can use a sale quotation template. You can activate the feature from settings.

First, create a service product named "Admin fee" and have a price on it.

Second, Go to Sales > Configuration > Sales Orders > Quotation Templates.

Third, create a new template or you can add your "Admin fee" service product in the default template.

Now, when you are going to create a sales quotation/order you will be able to select the quotation template. By default, the 'Default quotation" template is selected.

More details: https://odoo-online.readthedocs.io/sv/latest/sales/quick_create_quotation_using_quotation_template.html