Britrail.net isn't the Britrail site - it's a reseller - ACP Rail International
The Britrail information site is britrail.com which links to the all the different agents that sell the pass (including ACP Rail International)

Reservations are free with a Britrail pass - but you have to do them in person with the pass. The chargeable service is probably effectively a separate reservation without a fare ticket for which there is an admin fee or similar. This is most likely the same charge you faced.