Atticus says" Don't fool yourselves- it's all adding up,and one of these days we're going to pay the bill for it. what does he mean?

This can be found in Chapter 23.  Atticus speaks these
words after the Tom Robinson trial has ended and Tom has been
convicted.

What Atticus says is that black people have been
abused (like Tom Robinson has been) for too long.  He says that the frustrations of the
blacks are adding up and that there will be a rebellion some day.  That is what he means
by saying that one day they will have to pay the bill.

He
hopes that the bill will not need to be paid in his kids' lifetime, but he says that
eventually it will have to be paid.