Hi, Oktay! It is too expensive and I think it should be alway expensive to ask a travel agency. Suggest you do with it in Guangzhou in advance. Why don't you go Exit and Division of Entry Administration office of Guangzhou Municipal Public Security Department in 155, Jiefang Nan Lu,Yuexiu District. Usually, they can finish it in three working days. You will go next week, won't you? Or you can make a phone call to Canadian's Consulate General if you need a certificate to show your reason for extension. It will be much much cheaper. Good Luck.

maybe in beijing or hangkong....