Millennials and Gen Zers have an addictive new way to buy stuff that would look familiar to their great-grandparents. “Buy now, pay later” is a type of consumer credit that really got going in the 19th century when Singer sewing machines were sold for a “dollar down, dollar a week.” But the modern fintech twist in “BNPL” is that it’s aimed at people making impulse purchases of fashion or jewelry or electronics rather than sofas or refrigerators.