Many hotels advertised prices do not include the 10% bed tax and 7% Vat tax. This is usually shown in small print on the bottom of the rate web page or brouchure. To avoid this problem look for 17% Tax included in the price.

Now about the joiner fee. This used to happen to me often as my wife is Thai.
Because my work has me traveling all around Thailand and my wife works for an architect firm in Chiang Mai we sometimes meet in Bangkok on weekends. When I book a hotel I say I need double occupancy.

When I check in at the desk alone I tell the desk clerk that my wife will be flying in late that evening from Chiang Mai. I will be meeting her at the airport, we will have dinner and visit friends before returning late. Now I never have a problem or pay a joiners fee.
Ning flies in Friday late and returns to Chiang Mai early Sunday morning. She travels with only her purse and a small overnight bag.

You can do this if you want someone to join you for the evening. A word of caution here and I tell all my single friends who visit Thailand to do this.
Tell the person who is joining you that you want their ID card. Wrap the ID card in a piece of letter paper and place it in an envelope. Put your name on the envelope and hand it to the desk clerk and have them put it in a safe box.
If you have a safe box in your room put it in there. The next morning return the ID card to the person that joined you. If you do this there is no way that person will rob you and it does happen more often than you think.

Most good hotels will take the ID card from the person that is joining you at the time you pay the joining fee. It happens all the time so they won't think much about it.

Be safe,
Randy and Ning