Currently any curler can sign up as a spare for any league at any club, even if they are not registered in a league at that club. It would be helpful to optionally require a fee before allowing users to sign up as a spare.
The fee could be set up as a product, and the spare list could require this product to be purchased before allowing a user to sign up as a spare. That way all the existing discount behaviour would automatically work. For example, a discount could be applied if the curler is registered in one or more leagues at the club.