Applying for business visa

I have a letter of invitaion and all the required documents including work permit and long term visa for Thailand. I am a Australian living and working in Thailand. I have air tickets to Shanghai 11th and return 14th to Phuket. Reason to visit is to attend the Changhai Boat Show as a trade visitor. Can I use a agent/travel agent to go to Bangkok and make my application and collect the visa. Not go myself? Or do I have to go in person? To get the invitation letter I already had to prove by documentation that I was living in Thailand.