ご参考

ヨセミテ国立公園にはＳＦから日帰りツアーしか経験がありませんが、催行日がEvery day from late May to early November when Glacier Point Road is open - conditions permittingの記述通りローシーズンなのでWalk-in purchaseでも大丈夫と思われます。
8:30発／13:30発　　シニア料金(62超)　往復 44$ 片道 23$ 今年度料金
http://www.travelyosemite.com/things-to-do/guided-bus-tours/
不安であれば”Check Rates”をクリックして予約されたら如何でしょうか。
キャンセル条件はFAQに記述の条件が適用されると思いますが。
With the management change, is the cancellation policy the same?
Yes, the cancellation policy will remain the same, which means we must receive your request to change or cancel your reservation at least seven (7) days prior to your scheduled arrival date for a refund of your deposit. For reservations related to special events, we must receive your cancellation or change request at least thirty (30) days prior to your scheduled arrival date.Cancellation or changes to Housekeeping Camp reservations may only be changed after we receive your deposit, and a $10.00 handling fee, per unit will be charged for each change or cancellation. If you fail to arrive on the scheduled arrival date, your reservation will be canceled and you will not receive any refund or credit. We strictly enforce our cancellation/change policy and do not allow any exceptions.
http://www.travelyosemite.com/landing-pages/transition/faqs/