That worked..
All I've done is add (* 1.1) to the pages that show the cart.
eg. ($ShoppingCart, $ShoppingCart->DisplayInfo("TotalPrice")*1.1)
Now it looks like this:
price quantity price
$10.00 2 $20.00
sub-total: $18.18
tax: $1.82
grand total: $20.00
Thanks for your help Dani.