Yes, I agree! The sugar bag could be easily overlooked as a bit of symbolism because it's so subtle... it takes a lot of looking to really notice what kind of bag that is.
Retropian, I really like the connection between the sweet life and the sugar bag... yeah, this is a turning point moment for Ennis where the possiblity of a sweet life is still within his reach. And, it most definitely seems important that he has it immediately after coming down from Brokeback.