you're on the right track, just need to take it a step farther.
on the calculations tab, create a new calculation as:
[MaterialsFee] + [Price]
and edit the Totalprice calculation from:
[Price] * [Quantity]
([MaterialsFee] + [Price]) * [Quantity]
In the cart display, change the binding that shows the price to show the truePrice instead.
Also, in the behavior that passes the item information to the gateway, you will need to edit it to pass the TruePrice instead of price.