Giving money back to a customers and automating invoice descript & price.

[75volvo at comcast.net]

I have collected sales tax from my customers and happily it turns out in my business I am not required to do this.  I want to give it back by crediting their next invoice but I don't understand how to setup an account to show during the invoicing to properly reduce the liability:sales tax account.
When creating the invoice "liabilities" shows in the "income account" selection and if I choose this and put a negative number in the "unit price" it seems to reduce the "liabilities" account but does not directly reduce the sales tax account.

As an aside my service is a subscription service and almost every month the invoice would read exactly the same (description and unit price).  Is there anyway to automate this part of the invoice process?

James