The standard $30 service fee is per one way journey - so $60 for a return journey.
For long haul it's $60 per one way journey.
Could you not book online with the voucher you received? Did you try and debate the issue at all when you booked? Contact centre staff in particular do have the ability in some cases to waive fees for products and/or services that can't be booked online. Booking in store or at an airport will always incur a fee.