The longer version
A booking commission is the marketplace business model. The platform charges the club little or nothing as a monthly fee and instead takes a cut of each booking it handles. It appears on the invoice as a per-booking line, a percentage line, or is deducted before the payout arrives.
Clubs care because the commission scales with success. A club that fills its courts pays more every month than one that does not, and the cost is easy to miss when it is netted out of payouts. It is also separate from the card-processing fee, which every online payment carries regardless of platform.
The mistake is comparing a monthly fee to a commission without doing the arithmetic on real booking volume. Take last month’s bookings, apply the commission, add the card fee, and put the total next to a flat monthly fee. Then check whether the commission also applies to members and to bookings made at the desk.