What is a single supplement? The single supplement is a surcharge to unaccompanied travelers.

Why is there a single supplement?   Let's say the cost to someone like The Traveling Professor for a room for 7 nights at a 3-star hotel with breakfast is $1,000 (it is usually more but I am using the $1,000 figure as an easy-to-explain number).   If 2 people occupy the room, the cost is $500 per person.   If 1 person occupies the room the cost is still the same but some hotels may lower the cost, at most 10% if only 1 person occupies the room.   In NO CASE will the hotel cut the price of the room by half if it is only occupied by 1 person instead of 2 travelers.   Therefore, if an entire trip is advertised as $2,000 (hotel + guides + tickets + other services) for double occupancy (2 people in a room), if only 1 person is going to occupy the room, the difference in cost per person needs to be made up.  Hence, the single supplement.   In the illustration above, the single supplement would be about $400 if the hotel agreed to reduce the room price by 10% if it was only occupied by 1 person.

What types of accommodations do NOT charge a single supplement?  To my knowledge, the only place where there are no single supplements are hostels.  True hostels charge by the bed, not by the room.  I don't know about you, but at this point in life I am reluctant to sleep in a hostel.

Is the single supplement discriminatory?   Well, it is unfortunate, but not discriminatory.    Tour operators need to cover costs. They would not be in business if they did not cover their costs.   A way to eliminate the single supplement is to charge those 2 travelers occupying a room more than their fair share to cover the costs of a single traveler occupying a room by themselves.  Is that discriminatory?

But aren't single rooms smaller than those rooms occupied by 2 travelers?  To minimize the single supplement, many tour companies put single travelers in smaller rooms, usually with less amenities and inferior locations or views.   The Traveling Professor listened to his travelers and no longer does this.  The Traveling Professor travelers get the same size room as those with 2 travelers.

Are solo travelers subject to other single supplement fees?  Now, that WOULD be discriminatory according to The Traveling Professor.   Single travelers should not pay any additional fees, other than those that actually incur an additional cost.   The Traveling Professor does not charge any other additional fees other than what is actually incurred by having a solo traveler.   As far as I know, there are no other additional fees than the cost of a room.

Is there a way to fairly eliminate the single supplement?  There are only 2 ways to eliminate the single supplement.  The first one would be to find a hotel that charges 50% of their double occupancy rate if only 1 traveler occupies the room.   In all of my years in the travel business, I have not found one hotel (some hostels will do it) that would follow that policy.   The other way to do it would be to offer a tour where EVERY room is a single room only, which The Traveling Professor has done in the past.