@Sha I typically cross at Bluewater Bridge in Port Huron.
To my knowledge they are still issuing I-94 cards and charging the $6 US fee.
Its great that you have not been asked, but I would keep it in mind and don't be too surprised when they do eventually ask you to pay.
The website below will explain the I-94 better than I can.
*Tip* You can also use this website to check out your travel history when you are completing your waiver in case you can't remember the dates you have crossed into the US.