More and more of the tolls are going cashless throughout the US, and they are getting harder to 'ignore'. On our route down I-81/77 North Carolina has toll express lanes through Charlotte, and all of the new lanes added to I-4 through Orlando are cashless. If you take I-95 through the Washington area it is the same. These lanes are right beside the regular lanes with controlled access. If the traffic is heavy you'll be tempted to use them! Example, on our Saturday return in March, the regular lanes through Charlotte were slow and congested. For about $4 in tolls we were able to sail past all of that.
And yes, they can catch up with you in Canada with a mailed bill. AFAIK, the vehicle registration database is integrated throughout the US and Canada, so they can match an address to your plate. Unlike Ontario, the toll charges are pretty reasonable; $4 to bypass Charlotte is a lot better than the $40 to bypass Toronto on the 407, although many US tolls are also demand based, so more in higher volume times.