When a product that uses an eDocBuilder template is ordered through Pressero, is it possible to know what session on eDocBuilder the Pressero order item is related to?

Yes, it is a little tricky, but can be done fairly easily. In Pressero admin, edit the order and go to the eDocBuilder tab for that order. Right-click on the Edit this Order button that shows above the preview on the eDocBuilder tab, and choose Copy Link Location.
Paste down the copied link location into a text editor. It will show a long string with a lot of data in it, like this example:
Copy out the portion that follows docSession= and ending at &postbackURL, as shown in bold in the example above. The SessionID value for the example order above then is: bf18e176-54af-4e82-9efe-5bbdca1e68bf
Then login to your eDocBuilder account. Go to the Orders tab, and enter that value in the SessionID field on the Orders tab. That is the eDocBuilder session for the order.