OK I thought this through more... instead of the surcharge calculation I think you should probably add a RealPrice calculation and update the TotalPrice.
RealPrice = ([Category]==2)?[Price] * 1.05:[Price]
TotalPrice = [Price] * [Quantity] * (([Category]==2)?1.05:1)
To display the decimals you can use number_format like:
<?php echo(number_format($rsPROD->getColumnVal("wbprodPRICE")*1.05,2)); ?>