There are 2 occasions where you may find an activity charge applies.
If you are on a pay as you go membership, once you have used your taster or if the class does not offer a taster, you will see the price to attend displayed at the time of booking. This will be charged to the card on your account when booking is complete.
There might be venues you want to join, but because they are slightly more expensive they are not fully included in your plan. You are still able to join, but you will see a top up fee applicable. If you book the activity you will use up a session and the fee will be charged to the card on your account.