Switch Formula on Multiple Selection on Slicer?

Hi All, 

I have a what I think, a pretty simple model/ask I am asking of Power BI (I think), and still can't figure out what I'm doing wrong. 

I have one fact table and a product line (category) dimension table. 

My current code is this:

VAR selection_lowest =  SELECTEDVALUE ( 'Product Line Filter'[Lowest] )

var SALES_GAL = [9775 - SALES_GAL]

var SALES_LBS = [9380 - SALES_LBS]

var SALES_LBS_ETH = [16034 - SALES_LBS_ETHYLENE]

var A = CALCULATE(SALES_GAL, FILTER('Product Line Filter',[Lowest] = "A"))

var B = CALCULATE(SALES_LBS_ETH, FILTER('Product Line Filter',[Lowest] = "B"))

return 

switch( 

   true(), 

   selection_lowest = "A", A,

   selection_lowest = "B", B, 

   selection_lowest in {"A","B"},  A + B,

0 )

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Context: Essentially I have a slicer (Product Line Lowest), that works for "A" and "B" on their own levels, however, when both selected they do not sum together as expected, but instead return a 0. 

Any help or direction would be appreciated!