Now that you mention it...
I don't usually group by aggregates, and I would modify the stored proc to give me those numbers if I needed to, but with that said...
The sum by customer won't work outside of the customer group will it...BUT you can use a formula to set the order in a group, so what I would try instead is to use the formula in the Sorting of the group, that should work...
which might imply that the first method would work. I would try them both, since it wouldn't take too long add a group, delete the group and modify the sort