Does this mean that your deeds---good or bad---only count for "in the moment"?
Depends what you do. Some deeds have far reaching effects, like the holocaust or Martin Luthers' separation from the Catholic church. But I don't believe anyone faces consequences for their deeds after their death.
Do you think the idea of "afterlife" was just a method to keep people in line thinking there would be a greater reward for "doing right"?
I believe this is only a small part of it, "Religion is regarded by the common people as true, by the wise as false, and by the rulers as useful." - Seneca the Younger.
I believe it originated much more in our natural fear of death.
If the majority of people did not believe in an afterlife and someone to account to...do you think it would be even more barbaric...like "get what you can" at anyone's expense? Or do you think people would treasure each other more knowing this is all?
Atheists commit less crime than believers. The American prison system is full of Christians.