If a foreign plate passes through ISTHA and the customer doesn't pay, what do they do?
They submit a plate ID request with the state's DMV to acquire an address. They mail the bill to the address.
However, some states DMV don't respond to information demands from toll highway authorities, just law enforcement.
So if a single plate racks up more than $500 in unpaid charges, a legal complaint is filed through the Northern District of Illinois for an unpaid collection.
Then the States Attorney's office files a legal demand with the states DMV for the current address. Once the address is acquired a demand letter is sent for payment, provide a written response or to appear in court.
The largest issue I have read about is that once the toll authority gets an address from the DMV, either on demand or through legal methods, it is usually not the current address and so nothing happens.
Several toll authorities have tracked significant unpaid tolls (ie: Pennsylvania) and marked them as uncollected.
As long as the uncollected remains a small percentage of overall revenues, they don't spend a large amount of resources on it.
But I have read about car owners reporting a stolen plate with their DMV when they get a demand letter.