If using Google Merchant Centre (formerly Google Base, nee Google froogle) you must send prices inclusive of tax and you must also display prices with tax on your site. Normally this isn't a problem as the googlefroogle.php looks up the setting for displaying prices with tax, and if that is 'true' then the tax is added to the prices in your feed.

However, if you have the Include and Exclude Tax in Prices mod installed, that has display price settings of 'false', 'true', 'inc/ex' and 'ex/inc' - but googlefroogle.php will ONLY add tax if the value is true...which gets you into trouble with Google eventually.

Two ways around this (assuming you want to use inc/ex or ex/inc) - either manually change the Display prices with tax to true before you run a feed, then change it back to your preferred inc/ex or ex/inc after the feed is created or, because that is really unworkable, edit includes/functions/functions_taxes.php. Find the function zen_add_tax (at about line 158) and then change
Code:
if ( (DISPLAY_PRICE_WITH_TAX == 'true') && ($tax > 0) ) {
to
Code:
if ( (DISPLAY_PRICE_WITH_TAX != 'false') && ($tax > 0) ) {