[AMM]

I would suggest investigating rate configuration along the following lines... (I'm assuming from your inquiry that you do not want the $2 extra carton charge to be applied for the cartons less than X?).
This might not be 100% correct, or the most elegant way to do it, but it's a start...

Use three rate costs, one cost as the "base rate", one cost for the extra charges and one negative cost. Use the "Add" multi-costs rule on the Rate Costs tab.

For the base rate - the rate basis should be ship unit spec = carton, while the amount should be per ship unit.

For the extra charges - the rate basis should be ship unit spec = carton and number of ship units > X, while the amount should be per ship unit.

For the negative cost - the rate basis should be ship unit spec = carton and number of ship units > X, while the amount should be a negative value of -($2 multiplied by X).

Thus, for an order with number of cartons < X, only the base rate will apply. For an order with number of cartons > X, the base rate will apply, then for each carton a charge of $2 will be applied. The negative cost will effectively remove the the $2 charge for each carton below X, leaving you with the desired cost result.

Alternatively, as I've been typing this, I realised that you could possibly do something more elegant and simple to maintain using rate unit break profiles applied to each rate cost to configure the carton costs above/below X. However, since I've only mostly used weight breaks, rather than breaks on ship unit quantities, I'm not as experienced here.

As always, prototyping your rates with a few sample orders (e.g. with number of cartons < or > X) is the best way to test whether the config works the way you want . (Also I've had issues in the past with my rating results with using the wrong choice from per "ship unit" or per "shipment total number of ship units" in the rate basis.)

Hope that helps a bit...