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The single supplement is a premium charged to solo travellers when they take a room alone. The amount involved ranges from 10 to 100 percent of the standard accommodation rate. Accommodation vendors justify the charge as an appropriate business practice. Single travellers see the charge as price gouging at their expense. Discounts for booking early or repeat bookings and programs that arrange shared accommodation are variations on pricing that can ameliorate the single supplement for solo travellers. Research prior to travel may find companies that have removed the single supplement. 
Vendors and agents in the travel industry quote package-deal and accommodation prices in terms of dollars per person when the customer travels in a group of two, that is, "twin share" or "double". The single supplement is a premium surcharge applied to a traveller who travels alone but will use a room that caters for two.
Rationale for the single supplement
Arguments for the supplement are twofold. Firstly, solo travellers should pay for the luxury and convenience of having a room to themselves. Secondly, accommodation vendors must be compensated for the cost of preparing a room for a guest (cleaning and provision of disposables) when only one person will be charged.
Objections to the single supplement
The solo traveller argues that travelling alone is often a necessity rather than a luxury and that the cost of preparing rooms for guests should be distributed among customers without discrimination, or (more extremely) that couples should be surcharged for the privilege of not having to sleep apart. 
Objections to the single supplement may be partly explained by the "framing effect" described by Nagle and Holden in 2002.  This effect occurs when customers see cost as a combination of gains and losses. Take as example, a hotel room advertised at #£300, with a £25 discount for early booking and compare it with the same room at #£250, with a 10% single person surcharge. Customers will tend to choose the arrangement even though the cost of £275, is the same. Customers tend to choose gains over losses. An early booking discount is seen a gain, whereas a surcharge is seen as a loss.
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