sure this can be done.
you would need to create a recordset on the confirm page that will filter the zip code entered on the zip code column of our tax table.
Set the fltering options of the recordset to use:
zip code column =
Form Variable shipping_postcode
Then add a set session value server behavior, on the server behaviors panel, click the plus button and select eCart -> General -> Set session Value.
set the trigger to use any form post.
set the name to taxRate
Click the lightning bolt next to value and select the rate column from the recordset.
You may need to arrange the code so that the set session value code is after the recordset.
in the tax calculation, set it to a flat rate of $_SESSION['taxRate']
