How do I apply a discount for the second and successive items only if the quantity for those items is higher than 1? Using Commerce Discount and Commerce Discount Extra modules there is no problem to set discount rule conditions for product type and quantity. This way I can set up 20% off discount that is applied for a cart line item when this line item has specific product type and quantity more than one. 20% off discount is then calculated for this line item and multiplied by quantity.
However, my need is to apply such discount not to each item from line item quantity. I simply want to exclude applying discount for one (first one) on the basis: 0,2*(n-1)*X where n is qty and X is product base price.
The problem is that line item discount type in Commerce Discount module is calculating before quantity calculation (only for qty of 1) and proper price component is added to line item price.
The solution I've tried was to divide calculated discount (0.2*X/n) but the result calculated for quatity 3 or 6 or 9 is inaccurate (wrong, decimal number).
Is there a hook or other solution that can allow me to &quot;return&quot; the part of discount that should not be applied, or am I simply missing something and there is a more natural way to achieve what I described (for example,  the order discount rule)?
You should be able to do this out of the box if you have the Commerce Discount Extra module installed. The offer type to use is Per-quantity product discount (% off). This offer type (and the other per-quantity offer types) was created to support BOGO style offers (buy 1, get 1 free), but we developed them to support any type of discount for any number of matching products.

In your case, you'd set the "Buy #" to 1 and the "Get up to #" to a very high number... whatever you consider reasonable for the quantity you'd expect to discount. This will result in the creation of a new discount line item.

Note that the pattern for matching products will be SKU based or product category based.