I saw on the hotel site that the room I booked can accommodate up to 5 futons, which gives me the impression that it's ok if I have the 3rd person in the room. The 3rd person who is my son, decided to follow us last minute , after I have booked all hotel rooms in Hokkaido, (which explains why I cancelled some rooms with Expedia earlier due to the hotels' policies not able to accommodate 3rd person in a double room). That was after I requested for extra bed and was turned down. Thi hotel, having staffs who speak little english, didn't explain to me much after I told him about the room stated can have 5 futons, so I didn't bother to include the 3rd person.. And the front desk staff just replied "ok, ok, ok...", and the next day when I booked out, I was charged another JPY8,300+ for the 3rd person, which is SGD104. And the hotel did not bother to add a 3rd bath towel, face towel, etc for the room. I find it's very unjustified on my part..
