In standard AX 2012 it is not possible to have negative discounts.

But it is a very small change to actually make it work.  Just change the extended data type DiscPct.

But why would any allow negative discounts ?

In many industries they have specialized on full ballet/box delivery.  If a customer needs less than a pallet/box, then a extra charge should be put on the invoice.  Then it is possible to create a line discount that increases the line amount.

But there are some aspects that you should evaluate before we start using this approach:

1. It is normally much better to use charges, but auto charges is dependent on items or quantity, so then this must be a customization.  Misc charges also have the effect to better control the ledger posting.

2. When using negative discounts, then ordinary discounts will override this.  So you are then suddenly stuck, because the real line discount is lower than the negative discount.  But this can be compensated by making sure that the real discount only kicks in in a specified quantity range. 

My recommendation is to rather extend/customize the auto charge functionality in AX 2012, and make it possible to be dependable on customer, item AND quantity :

Maybe an ide for next release of Dynamics AX ?