They do take something like 10% on some, but not all, bookings now.
Interesting, that must be very recent.
It is 12% but capped at £1.99:
TrainPal charges 12% of the difference between the original price and the SplitTickets price with a maximum SplitTickets service fee of £1.99.
TrainPal Help Centre: Check out frequently asked train tickets booking and tickets management related questions.
www.mytrainpal.com
It does make reference to a general booking fee but I've never seen it come up, and can't work out under what conditions it would.
In that sense it falls into a
similar camp as Trainline, where their fee is capped at £2.99, split ticketing or not.
In that case I revise my suggestion to ScotRail, who as of July also offer split tickets on their website and app and charge no fees/commission.