This is where is gets difficult, because usually this sort of information isn't stored in the server logs.
This sort of information is going to be in your transactional database, so the tricky bit is to interface the two sources of data.
Top end analysis apps tend to either store session/path information in your database or store transaction information in the logs (possibly by calling a 'bug' graphic with a transaction ID attached as a dynamic parameter, or a thank you page in a similar fashion).
If you choose the former option, a simple database query would suffice, and you could avoid log analysis completely. With the latter option, you should be able to use even simple log file analyser to pull out the data you need.
If all of that sounded like a foreign language, you may want to check out Clicktracks and/or Urchin.
Edit: Square brackets!