I'm having an issue where our client's customers are getting charged extra shipping when their cart contains products from multiple categories.
Our site is setup for multiship and we're using flat rate shipping ($12) for all categories. On one specific category, "Candy", we need to charge a $10 handling fee. I currently have the "Candy" category set up with $12 flat rate shipping (charge per shipment) and $10 handling fee. I also have "Default (all products)" set up as $12 flat rate shipping (charge per shipment).
If the customer has ONLY products from the "Candy" category in their cart, they are correctly charged $22 for shipping. If they have something from "Candy" as well as, say, a coffee mug in their cart, they are being incorrectly charged $34 for shipping - should be $22.
What am I doing wrong here and how do I fix it?