After buying an item internationally customs intercepted the package and was asking in its declaration form for the PayPal receipt if applicable. Falsifying such a receipt would be beyond easy, so, PayPal being an officially registered money service, does that mean they can next easily check with PayPal whether the provided information is valid (e.g. look up the transaction number in question)?
I presume that if they can this will likely be internationally handled to some extend as part of the greater international financial system that allows for international transactions, but if it's not then a distinction can be made between the US government and any government in general.
Just for the record: I am not interested in falsifying anything, I am just from a privacy point of view interested whether customs could access this kind of data.