Tableau - Bins based on Fixed LOD Customer Units

Question

What I'm attempting to do is create a histogram based on a Combined Field (Child + Zip Code, which is our definition of a customer) that shows the distribution of customers depending on their yearly ordering size (1-10 units, 11-50 units, and so on).

Problem: I can't seem to figure out how to appropriately compute the different bins. I've seen a lot of Tableau blogs on utilising bins, but none that computed based on a unique id like mine. If a customer's unit sales exceed the, it appears that they are placed in every category (1-10, 11-20, etc.) rather than a single category. Maybe I'm misinterpreting FIXED LOD calcs.

To show on a histogram, get a count of the consumers in these distinct ordering ranges.

This formula isn't working:

IF { FIXED [UID_Cust] : SUM([Units]) } <= 10 THEN '1-10'
ELSEIF { FIXED [UID_Cust] : SUM([Units]) } <= 20 THEN '11-20'
ELSEIF { FIXED [UID_Cust] : SUM([Units]) } <= 50 THEN '21-50'
ELSEIF { FIXED [UID_Cust] : SUM([Units]) } <= 250 THEN '51-250'
ELSE '>250'
END