Agoda booked a hotel in Thailand. How much is the tax?

Taxes and service charges are hotel or local regulations, and have nothing to do with the website. So just pay attention to the total price.

According to the time stipulated in the website, it should be no problem to modify the itinerary, and there is no extra charge, unless there is no room of the same level on the date you want to change.

However, from personal experience, agoda is slightly more expensive than travelasia and booking.com. But their service is not bad.