Lots of day-of options around the stadium. Some lots presell parking passes, but usually save some for day of parking.
There are plenty of surface lots on the South End side (SW of the stadium between Mint St and South Blvd), just NW of the stadium (along Morehead and Cedar St), and directly E-NE of the stadium (along 4th/Graham/Mint streets). They'll probably have ability to pay with a card, but I would carry cash just in case. Usually anywhere from $30-80 per car. Plenty of options