I think that language on the confirmation sheet must refer to no-shows or maybe to a pre or post tour.

In the booking area online there is a whole long paragraph of terms and conditions that you have to check that you agree with, including a 36 hr cancel period:
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In order to ensure quality guides and sufficient transportation, we must provide the tour operator with passenger counts at least 36 hours in advance. Therefore no refunds will be given within 36 hours of tour departure for cancellations or no-shows. Please pay special attention to scheduling and tour lengths when booking your shore excursions. In many cases, it is not possible to participate in more than one tour per day in a given port. Tours operate rain or shine. Tours require minimum participation levels to operate so please sign up as early as possible. Tours may be cancelled if minimum participation levels are not met. Prices, Itineraries and General Information shore excursion prices are subject to change to meet unexpected cost increases or currency fluctuations. The information provided here is current as of today. However, changes may occur before your scheduled departure. Please consult the Destination Services Desk onboard for information and current pricing. During local or national holidays, access to certain facilities (such as museums or archaeological sites) may be limited or denied. In such instances, adjustments will be made to minimize inconvenience to guests. Some tours have limited availability. Please sign up early to avoid disappointment. Regent Seven Seas Cruises reserves the right to require guests to complete a liability waiver for select shore excursions. For more information, please visit the Destination Services Desk onboard. To make changes to tours already purchased, please contact Regent Reservations at 877.505.5370

Accept Terms
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