inCart is used to lookup an items ID to see if a particular item is in the cart. The argument that inCart Accepts is and ID number:
InCart($itemID)
you cant pass it a column name.
For what you are trying to do, use the Conditional Total function, the structure for the Conditional total function is:
ConditionalTotal($ColumnName, $ConditionalColumn, $ConditionalValue)
so the if statement should be:
if ($WebCart->ConditionalTotal("Quantity", "ItemType", "2") > 0) {
this will return the Quantity of items where the itemType column is 2, if that is greater than 0, write the shipping submit button.