I called and was able to reach the helpdesk early this morning. Here's how you do it:
Go to REPORTS ->Transactions -> Transaction Finder -> Enter key words (I entered "USPS" because they're who I ship through). They will only give you 90 days at a time, so I had to run five reports.
At first it doesn't appear as if there is a total for that time period, but if you hit (what I am calling the fast forward icon) and scroll to the bottom, you're total for that time should be at the bottom.