When the first automated check-out machines were tested, they weren't immediately popular with customers; but while today's versions are still far from perfect, the proliferation of self checkout machines is undeniable.According to Allied Market Research, the self checkout industry is projected to garner $31.75 billion by 2020, which could bode poorly for the future of cashiers. Popular services such as Amazon Fresh also allow shoppers to order groceries and other items from the comfort of their home.