We have 4x Maxi-Cosi RodiFix high-back booster car seats available as our kids have grown out of them.
All in good condition. Can provide pictures and other information if you like.
This is one of the narrowest high-back booster out there making it easier to fit into the 2nd row of smaller cars. They were $220/ea new. Willing to part with them for $50 each OBO.

More information on the seat: https://www.thecarcrashdetective.com/maxi-cosi-rodifix-review/