You wish to add a tax/surcharge to all transactions - both your credit card and non-credit card transactions.
You can add a tax or surcharge to your transactions by filling in the Surcharge Name and Surcharge Rate fields at Tools -> Payment Settings.
- Surcharge Rate: A surcharge such as a tax or handling fee can be applied to all orders. You can add a Flat Amount, or a fixed percentage from 1-15%
- Surcharge Name: The name of the surcharge such as tax, handling fee, shipping etc. The Surcharge Name will appear on the order receipt.
- Surcharge Value: The value used to calculate the surcharge applied to the order. Depends on the Surcharge Type setting either a flat amount or percentage from 1-15%. Please do not use any symbols here, only numbers.
There is a similar field which charges a percentage to the order for credit card transactions only. Please see the article on adding a transaction fee to cover credit card merchant expenses. Note that you will need to use only one or the other of these fees as they cannot be combined.