This took a little bit of code reading, but once I found it, it makes a lot sense. So, let's start at the beginning. We're looking at a specific entity, "Order." You can manage the display of Orders by navigating to admin/commerce/config/order/display and there you can change the view that lists the line items, tweak the order total field, and any other customer profiles you might have defined.
To override with lots of customizability, this was Ryan's advice:
"That path is defined by the Order UI module. The output for that page is just coming from the normal rendering system for the order entity, which works just like nodes. Probably the easiest thing to do would be to implement a .tpl.php for order pages; that should be possible - or else he can hook into hook_commerce_order_view() and fiddle with the content array + add CSS."
If you wanted to go the .tpl.php route, Devel Themer (http://drupal.org/project/devel_themer) provides this suggested template name: page--user--orders.tpl.php and is using the themes/bartik/templates/page.tpl.php file by default.
EclipseGC responded to the issue of Page Manager not letting you override: "The order url pages won't be usable until a task plugin has been provided for it. Page manager needs a custom plugin to override existing hook_menu defined routes."
Hope that helps!