I'm trying to charge a flat "per cart" shipping fee for only a subset of my products. I created a "flat rate" warehouse and assigned the products in question to that warehouse. Then I created a Shipping by Weight configuration record with an "additional fixed cost" and assigned the shipping by weight record to that warehouse. Unfortunately, this fee never shows up in my cart after adding products which are shipping from that warehouse.
If I remove the warehouse assignment from the shipping by weight record the fee is applied to every cart, even if none of the items in question are included.
The warehouse filter in the shipping by weight plugin does not seem to be working as expected.