optional fee

例句：

1. 1. The hotel provides a free breakfast for all guests.
   酒店为所有客人提供免费的早餐。
2. 2. The admission fee is included in the total cost.
   入场费已包含在总费用中。
3. 3. Some activities have an optional fee, which can be paid separately.
   某些活动有可选费用，可以单独支付。