Categorize Sales Orders by Quantity on Order

I'm trying to build a workflow that helps our warehouse categorize their orders by size.  Amount is an available filter on the print pick tickets screen, but quantity committed is not.

Ideally, I want to have the following happen:

On submit of sales order, if total quantity committed < 4 units, then assign custom field x value. 

If > 3 units and less than 10 units, then assign custom field y value. 

...and so on.

I cannot get this to work.  I have been duplicating a workflow that I had help designing, but to no avail.  I have heard that workflows do not work at the line item level and you need a search to define the criteria here to make the change - but can't get that to work either.  I'm fumbling around here trying to figure out how to make this work.  I would prefer not to open a support case every time I run into a little trouble...the help center has not yet been useful..