Adding additional charges to a line item

[davist11]

We are currently building a Commerce site with some complex pricing rules. One thing we have to account for is adding additional flat charges based on a specific product option.

For example:

If a user selects Gold Irridite as an optional finish, we want to add a flat $150 charge to the line item, regardless of quantity:

The way I was doing this was by hooking onto the LineItems::EVENT_POPULATE_LINE_ITEM event, checking the option and then adding the additional amount to the line item price. Since this is the value of a single line item, I need to take the $150 and divide it by the quantity so it calculates correctly per item. This is working great when 150 / {qty} gives a round number. But if I submit a quantity like 499, then the additional amount that I'm adding per line item is 0.3006012, there's a rounding issue, and the price is incorrectly calculated

That should be $184.93.

This seems like the perfect use case for an adjuster, but what I need to add the additional charge for doesn't fall under an adjuster category

An adjustment model should have a type of shipping, discount, or tax.

Is there another way to add a flat rate charge that I'm missing?