It is possible to the App to return with a Sales Tax percentage based on your current location.  Even if you do not know the postal code for that location, it can get it with Location Services.  There is a button with a Pin Icon.  Tap that button, and your iOS device will get your current location and add the postal code on the "Postal Code" field.  Once you have that, it will automatically add the sales tax to your Item List.  If the Sales Tax does not update, tap "Refresh".

If you are looking to get the sales tax for another location, then enter the postal code manually on the "Postal Code" field.  It will update automatically and add the new Sales Tax.  If for some reason it does not update, tap "Refresh".