I can ship 70 pounds with an airline going over the counter. They will
accept a box that is no more than 90 inches overall and weighs no more than
70 pounds. I have a box that is 36x22x22 which is 80 inches overall.

I would like to calculate how many smaller boxes I can put in the larger
"Master Box"

I think I would need to answer four questions before doing the calclulation

What is the average weight of the packages being shipped?
What is the average length of each package
What is the average width of each package
What is the average height of each package.

For this example lets say the average weight was 1 pound
Average length = 4 inches
Average width = 2 inches
Average height = 3 inches

How many of these packages could I get in the "Master Box" before getting to
the 70 pound limit?

Any help would be appreciated.