Override tax to only tax certain amount (ex: max tax of $500)?

[inktopress]

My city in Alaska is wonky - it is the only city in the borough that charges sales tax (7.85%), however that tax only applys to $500 per transaction.  

Is there a way to override taxes to only charge for the first $500 of an invoice?  I need to invoice for $600, but must only tax on $500, but shopify is applying tax for the full amount.

Any help is appreciated!

Thank you,

Matt