Magento

Need a built-in feature which allows the shop owner to give special discount from the grand total,i,e, directly minus a custom value from the total value.

For example,there is an order:
Subtotal:  $856.49
Shipping:  $10.96
Tax:  $52.05
Grand Total:  $919.50
A box that you can put any figure into it and minus the figure directly from the grand total.

If customer wish to get the product at a total cost of USD900, you just need to put USD19.50 in the box through the order detail section in the back end,and the grand total turned to be USD900.

Please,Magento dev team,it\’s really a must have feature that every shop owner wants! Only this is a straight forward solution.Forget all other cart rules,promotion coupon codes and percentage discount etc!