Allen,Your services are excellent and that is why I always choose your hotel every time when I am in Beijing
May I answer your question instead. You are right to use present tense if you still think the hotel provides good service, which does seem to be so at this stage. I suggest you change to "Your service had been excellent, and that is why I always chose to stay at your hotel whenever I come to Beijing". You are complaining about their service, and you want them to feel like if they don't response to the issue properly, your view of their hotel will change and they will lose a regular customer. That's why past tense makes a stronger message in this situation.
This sounds a bit strange. How about we change it to: "It was 36 Celsius on that day, which was quite unusual in this season." You need to use past tense here, because you talk about what already happened.The temperature of that day was 36 degree, which is a quite unusual in this season.
Please remove "so", it is not necessary. Besides, you need to capitalize the letter "s" if you start your sentence with "so". Also, remove "for" after "called".so, I tried to turn on the air conditioner right after I entered the room but it did not work at all. I called for your room services immediately, but I was told that the air conditioner was not supposed to work until June.
Just few comments, hope they are helpful. Just note that there are still some awkward sentences, I just couldn't go through your whole letter.
Good luck, Allen.