Make Payment Surcharge Editable - Amazon And Ebay Collected Tax

The states in USA are now requiring Ebay and Amazon to collect tax. We have been adding the tax as a product but it is messy to extract this for tax reporting purposes. We can't set-up a real TAX because our website users will be affected by it. It looks like Payment Surcharge would be the best place to use this because the payment can be disabled for the website users but admin could still use it for manual Ebay and Amazon order entry. The problem is that this field is not editable in admin order entry. Has anyone made the Payment Surcharge field editable in admin. Any other suggestions are appreciated.