Seems a bit high but you can not rent a decent bungalow in Vientiane for less than $400 a month. A nice furnished 1br apartment would be the same. Laos may be poor but this is a capital city with lots of falang with good jobs.
•Assemble the final application for permanent residence, including applications, supporting documents, and proof that the medical examination has been done.
This is from the link you gave. A receipt or other indication would seem to cover it. The doctors cannot release information you may change the report.
Doubt if you can do.........Bangkok has no real way of checking your credentials........They are tough so you likely to be rejected unless very rich and well connected