In mining/mill towns (any town owned by a company) there was a common practice of paying workers in "scrip," valueless pieces of paper that could only be redeemed at
the company store, which often charged outrageous prices, which
exacerbated the cycle of
wage slavery.
Hence the song:
You load sixteen tons and what do you get?
Another day older and deeper in debt.
Saint Peter, don't you call me 'cause I can't go,
I owe my soul to the company store.