First, consider sorting by a customer ID instead of a name. Sorting by name might combine data from two different customers with the same full name into a single total.
Second, if you open the Sort Records dialog, you'll find that you have an option to "Reorder based on Summary Field" near the bottom of the dialog. You can use that option to rearrange your groups of records such that the Largest (or smallest) summary field value is listed first.
Awesome! Thanks. Didn't think it was going to be that easy!
I am actually sorting on the ID originally to avoid the unwanted grouping of customers with the same name.
The "re-order" can be slow so don't expect lightning fast results if you have a fair number of records in your report.
A hint for other reports when you want to group records by name and keep them in alphabetical order. (Which I realize is not what you want here.)
Sort your records first by Name, then by ID. Set up your sub summary parts to refer to the ID field as the "sorted by" field.
This produces an alphabetic order to your groups, but two people with the same name still get grouped separately and get separate sub totals.