It's not the outdoor centre that needs to afford the goodwill fee, it's the client! - and if the cost of sending Little Jimmy caving increases from, say, ?15 to ?18 then there's many who would simply decide to do something else instead.
However, all the Outdoor Centres will clearly require receipts for the goodwill fees for their own accounting, as a matter of course and, on requesting same, it may subsequently transpire that a reduced* non-receipted goodwill fee is acceptable, after all. Why not check it out.... you never know.
* Possibly even equating to a lesser figure than the original goodwill fee used to be. Fingers crossed.