Avoid upcoming problems and/or scams
It is common in Thailand that the full payment is divided into two or three-part payments. This is fair for both buyer and the contractor. a 50/50 split is most common for smaller amounts, around 10-99.000 thousand THB. If the total amount exceeds a certain sum, like 100.000 or more a fair split would be 40/40/20 to be paid "Before work starts, midterm, and when finished and approved by the customer". NEVER pay up 100% before work starts. The cheapest offer you get is always suspicious, especially if it is "too" cheap. Here is what could happen and often do: You chose the cheapest alternative, being a cheap Charlie. The contractor starts to work. After the contractor worked for a while he/she tells you that money is not enough to complete the work done. You oppose this, of course since you have an agreement. This will end up with the contractor leaving and never coming back. They did work already with a good profit. Nothing you can do about that. Now you need to get a new contractor which is not easy. Serious contractors do not want to take up a job done by someone else, to risky. If you manage to find someone willing to complete the job then do not be a Cheap Charlie again, you could end up with the same problem one more time. In all, you probably by now have to pay more than you would have if you chose the more expensive offer at the beginning.