Sort data based on value in a node

I can't seem to find any solutions on here already to this problem, even though I'm sure there are plenty.

I'm pulling data from a web service (name, date, email, company, subtotal), with subtotal being the most important for this problem. Right now it sorts based on the date the order was placed, but I need it to sort by subtotal (highest to lowest).

I can't make any changes to the xml or web service itself.