The fixed fee, not the rate
On a $12 wash, a fixed per-transaction fee is a far larger share than any realistic difference in percentage. That line is where the money is.
It is also the line most often left in place because the percentage was the only number discussed.
The equipment lives outside
Bays are wet, hot and cold, and the terminal has to survive it. Unattended outdoor hardware is a different specification and a different price from a countertop device, and replacing it is a different conversation too.
Worth establishing who carries the cost when a unit fails in service, before you need to know.
Downtime is lost revenue, immediately
With nobody on site, a terminal that stops taking payment simply turns customers away until someone notices. Uptime and fault response are worth negotiating as hard as the rate.