All it takes is you sending a picture postcard using the US Postal Service during your 'crime' to make it a federal thing.
This sort of hack to elevate crimes to federal level has been abused plenty of drugs cases in the past, so that separation of state and federal powers is mostly a matter of decorum, if the feds want to be involved in a case they will be.
The postal service being used within a state should not make a non-federal crime a federal crime. "Not crossing state lines" versus "using a service that could be used to cross state lines".
This sort of hack to elevate crimes to federal level has been abused plenty of drugs cases in the past, so that separation of state and federal powers is mostly a matter of decorum, if the feds want to be involved in a case they will be.