Tesla,
I haven't had time to test this, but you can give it a try.
Open the following file:
includes\modules\order_total\ot_local_sales_taxes.php

at about line 170 look for the following:
$debug2 .= $myfield."<br>";

right above that line place the following code:
PHP Code:
if($taxmatch == "postcode"){
$myfield = substr($myfield, 0, 5); 
} 
See if it does the trick.

LadyHLG