Had the same thing happen with Priceline just a few weeks ago, except instead of $20, the price was double the advertised price and only changed when I entered my payment info and tried to pay. I finally called and booked over the phone after having three different rooms double in price when I hit the confirmation button. The person on the other end must have really wanted to make the sale at that price, because they booked me for a month later than my request as the advertised price for my days seems to have just straight up not existed.

It was a pretty hellish experience trying to get them to change the dates, but in the end, they knew that I had a confirmation with my dates and I had paid, so it really would be fraud to not give me the room. Priceline: only once.