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Sorry but we do not have features for adding taxes to the products. You may however create a separate product to your payment field, label it as "Tax" and then make it required so users will be obliged to pay the included tax.
Here's how to make the product required.
Let us know if you need further assistance.
Thank you for getting back to me.
Would the fact that I have set up domestic tax rates in PayPal "My Selling Preferences" have any bearing on this?
Yes, that should work as well. We actually recommend that when someone would like to include tax rates to his Paypal forms aside from the workaround provided by my colleague (which is a fix rate).