How to apply a lump sum discount to an order without reducing each line price?

Have a need to quote for a lump sum bottom line discount that will be deducted off the subtotal of an order.

Line item pricing needs to remain intact without spreading the discount.

Can this be done within Advanced pricing or is it custom or Trade Mgt?

Basically a selling incentive that needs to be reflected as a lump sum discount off the subtotal for invoicing purposes to match what a customer was quoted as a lump sum bottom line discount, not a reduction in each line on the order.