How do I decline a $30 office gift contribution when the collection list is public?

The message said, "Suggested contribution: $30. Put a tick next to your name once you've paid."

Our team is buying a leaving gift for a manager. There are twenty names on the shared list, and most already have ticks. I like the manager and would happily write something in the card. This month, though, $30 would come from money I've set aside for my own weekend plans.

The organiser says the amount is optional, which I believe she means. But a blank beside my name will be visible to colleagues who don't know whether I couldn't afford it, forgot, or chose not to join. I could give $10 privately. I'm not sure whether offering less would create more awkwardness than simply declining.

Would you ask the organiser to keep payment tracking private, send a smaller amount without explanation, or leave your name unticked? I don't want to make the gift collection a debate in the team chat. I also don't want 'optional' to mean paying to avoid a visible blank.