Client wants to add a fixed "convenience fee" at checkout. Has anyone added a convenience fee to the checkout process? I considered just renaming the Tax field for that but then the actual merchant will show it as tax paid instead of a convenience fee.
This depends on payment system thatyou will use. For most payment systems tax just will be added to payment amount and user will not see that this was "tax".
You can use Ken Gary's amShipping for this. You'll get two additional fields in the product area where you can specify an extra amount to be added to national and international clients. In the config settings you can set a default value. Note that there will not be an extra sentence/line on the invoice, the amount will be added to the taxes. So you change to words to something like: "tax and convenience fee".