This will keep you amused – maybe and maybe not. It will depend on how long you will be working with a charity and what work you will be doing. You may not need one at all – check with your charity is the best advice.
You cannot apply for the work permit until your are in Cambodia
However the good news is that it is relatively easy to obtain a work permit should one be required.
The cost will be anything between $200 and $300. The good news is that you can continue with your charitable work whilst the application is in progress.
From personal experience, I have twice required work permits and the process was simple and efficient. However, the physical work permit can take a while to arrive.