For that number of trips, there is a cheaper way to reserve in advance from the US. There's a different agency that charges a single fixed fee for all of your reservations plus the standard Europe reservation fee.
I'll do a bit of research and report back in this post when I find that agency.
PDH
EDIT: OK, I'm back with more info...
The other agency is EurAide,
http://www.euraide.de
They charge you the actual price in Euros plus US$45 for shipping and handling the entire order. Their US (Florida) contact is at 914-480-1555. Have your trip details written down before calling - they may ask you to fax or email your info to them.
Here's their "how to reserve" page.