If you take it back to the tinter, they'll typically cut out a section for you to attach a toll tag if you want (same way they presumably left an untinted area for your registration and inspection stickers). Or you can just get an Xacto knife or whatever the tinter recommends if you want to DIY. It's pretty easy to remove tint as required, as opposed to adding more. I actually won't be needing one of the stickers on my windshield anymore (parking sticker for a place I'm moving out of), so I'll have a larger untinted gap, but having had Air Blue for a while myself now with a larger gap than necessary over the inspection sticker at my request, the untinted space doesn't ever catch my eye -- and if the untinted space for your toll tag is behind the mirror, you won't even be able to see it.
'16 Cayman GT4 (delivery pics
Gone but not forgotten:
'11.75 M3 E92 Le Mans | Black Nov w/ Alum | 6MT (owned 5/2011 - 11/2015)