As I understand the procedure, I must send my travel itinerary to the thai consulate with my arrival/departure dates for a 60 day tourist visa.

However, I would like to extend my stay when I get to thailand to 90 days.

Would the consulate reject my visa application if I submitted a travel itinerary with a 90 stay because I am applying for a 60 day tourist visa?

The reason I am subitting a 90 itnerary is to avoid the added cost of changing my flight schedule when i get to thailand from 60 to 90 days

you will get 60 days when arrive, on the last day go to any immigration office and you will get 30 days extention for 1,900฿...