For WSF:

Handling Fee Per Model: White: $1.50, Black $1.75, Polished: $2.00, Dyed: $2.25

Price Per cm3: White: $1.40, Black: $1.75, Polished: $1.50, Dyed: $1.50

For models that are greater than 10% dense (material volume divided by bounding box volume), after the first 20cm3, the remaining volume is calculated with a 50% discount.

If you go to your model and look at it in Edit mode then it should tell you the volume of material and the dimensions of the Oriented Bounding Box. If you calculate the volume of this OBB and divide the actual volume by the OBB volume this will tell you whether or not your model qualifies for the volume discount.

Using all the above (and a spreadsheet!) you should be able to calculate the theoretical cost of each of your models.

What you have already stated does not seem to make sense, as it seems to me that the single model should always be cheaper than the twin models:

a) Because you are saving the handling fee of one model;

b) Because if you qualify for the volume discount, then in the twin model case you will have to pay full rate for up to 20cm3 more than in the other case;

Perhaps the cape is the problem - maybe it is increasing the OBB to the point that you get no discount. As a test, you could lay it flat underneath the other components (I understand that you would not want to do this for orientation reasons, but just for a test).