How did Atticus's remark that "it's all adding up and one of these days we're going to pay the bill for it," relate to the civil rights struggles of the late 1950's and 1960's when this book was written?

This is a great question. Atticus speaks these words after the Tom Robinson trial has ended and Tom has been convicted. Atticus is speaking about the centuries of abuse that black people have been handed by white colonial powers. Maycomb and the American South is an extension of this abuse. For every action there is a reaction and certainly the pot has been boiling for a very long time. America will have to pay it's debt for the abuse one day. There are ominous implications to this. This debt could very well be paid in blood if the racism and inequality did not change. We certainly see this in the Civil Rights movement that you mentioned. Martin Luther King was able to keep a lid on the violence as best he could but something had to give. Either a radical shift in racial policy was won or there would have been some kind of reckoning. Atticus certainly was not a prophet but he saw things in a rational way. He was crushed over the ridiculous verdict handed to his client and he knew that one day a price would have to be paid.

I dont know the answer, thats why i got on this website.

Tom was convicted.