How does the U.S. Overflight Fee system work? After going through the Overflight Fees on FAA.GOV, the following is clear and self-explanatory:


Exact fees
Method of Payment (online, via Pay.gov as mentioned on the page).


What's not clear to me is the following:


  The FAA charges overflight fees to operators of aircraft that fly in
  U.S.-controlled airspace, but neither take off nor land in the United
  States.



Does this mean that international flights that land at any airport within the U.S. are not liable for overflight fees, and that they're only liable to pay the other fees such as airport charges etc.?
How is the invoice/bill delivered to the responsible party? I
believe airlines/commuters have accounts that get billed, but what
about private jet owners?
What would be the circumstances if, say a private jet owner/operator
departs from Canada, lands in Mexico and doesn't pay the invoice?
What if such a trip is made once a month for a long period of time
without paying the due fees. Does that jet "not get permission" to
enter U.S. Controlled Airspace?
Correct. Some countries charge overflight fees regardless of whether you land or not. In the US, overflight fees are charged per 100 nm. You can find the exact fees for 2017, 2018 and 2019 here. The fees can be paid online, and the FAA is pretty modern when it comes to this; it accepts ACH transfers from a US bank account, Amazon account, Dwolla account, PayPal and Debit or credit card.
The aircraft has to identify itself. The flight plan tells the FAA who you are. They look you up in the registry of the country in which your aircraft appears and send you the bill.
Unpaid fees accrue interest, and can result in the aircraft being denied entry into US airspace or impounded if it does happen to land at a US airport. The FAA can also put liens on the aircraft in the country of registration, which would have to be paid before the aircraft registration changes hands.