Custom Tax Amount Field Under the Tax Option

Please add a "custom tax amount" field under the "select tax" tab so that we can add our own tax amount if need be. I have some vendors who tax only on product, but not installation. If the invoice is lumped together it's very hard to break the tax out. I added it as a line item on my invoice to my client, but then it gets flagged in DF as income vs. tax and messes with the books. My bookkeeper got very confused as I should not be charged tax on my client's tax. Please reach out with questions as I now this will help others, as well.