The $10 in free SR 520 tolls was a limited time offer that is no longer available. 

The SR 520 toll credit customers received for activating their Good To Go! Pass early last year expired on March 31, 2012.

More on how the $10 is added to your account:

Your account will be credited back for all SR 520 Bridge toll transactions until the $10 credit has been used, or until the promotional period ends on March 31, 2012.

The credit is applied when a pass is used, up to $10 worth of tolls will be credited per pass used. If a pass with a credit does not cross the SR 520 Bridge before March 31, 2012 the credit will expire.

Details:
The toll credit has always had an expiration date. Under the original terms of the program, when tolling was expected to start in April 2011 drivers had approximately two and a half months to use the credit. The expiration date of the offer was adjusted when tolling started so that customers had three months to use the credit.

• Passes must have been activated by April 15, 2011 to receive the offer.
• Toll credits are only valid for the SR 520 Bridge tolls and must be used by the end of the promotional period - March 31, 2012.
• The credit will post to your account at the end of each month for SR 520 tolls (after tolls have been accumulated) until the entire credit amount has been used. The credit will be shown on your monthly account statement.
• You can receive $10 in free tolls per Pass, up to six Passes, applied toward one account.
• Promotion applies only to new Good To Go! accounts.
• Offer is valid on registered Good To Go! Pass accounts only (commercial accounts, Pay By Plate and unregistered accounts are ineligible).
• You must have registered your Pass(es) and activated your account with the minimum $30 opening balance in order to receive the $10 toll credit.
• Toll credit has no cash value and cannot be refunded.