These shops can be seen around most big stations and in cities.
They buy discounted tickets in bulk and resell them to individuals at a cost 5-10% cheaper than the regular price.
They’re often referred to as ‘kinken shops’ which means voucher/coupon shops.
They’re identifiable by their white signs listing destinations and prices in red.
They may look a little shady but they are totally legit
For some more detailed info, check out cheapo Japan’s site.