Invoice Drop Size with Slicer

hi, danb 

After my research, you could try this way as below:

Step1:

Use a What if parameter to add a "Drop Size Threshold" slicer

https://docs.microsoft.com/en-us/power-bi/desktop-what-if

or you could custom a "Drop Size Threshold" slicer table and add a Parameter Value measure

Parameter Value = SELECTEDVALUE('Parameter'[Parameter])

Step2:

Then add two measure

Average Drop Size = CALCULATE(AVERAGE('Table'[Total Cases]),FILTER('Table','Table'[Total Cases]>=[Parameter Value]))

# of Drops = CALCULATE(COUNTA('Table'[Invoice #]),FILTER('Table','Table'[Total Cases]>=[Parameter Value]))

Result:

and here is a pbix file, please try it.

Best Regards,

Lin