Actually, the current code allows a lot of flexibility. The Misc Cost will accept positive and negative cost adjustments. When the cost adjustment is negative, a credit is applied, and the tax is reduced by the tax on the cost adjustment. For a positive cost adjustment, the total increases by the adjustment plus the tax on the adjustment. Neglecting quantity and group discounts, this behavior is the same as increasing or decreasing the price of a taxable product.
Unfortunately, the one-time discount adjustment always calculates a discount for both positive and negative adjustments. If the sign were honored, you could use one-time discount for after-tax (non-taxed) credits and after-tax costs.
Because I want the flexibility of both before and after tax credits and costs, I'm going to search for where the one-time discount input is processed and see if I can get it to honor the adjustment sign. Rewording some of the displayed text may also be helpful.
Dave