From Affirm to Klarna, buy now pay later startups are booming. But experts warn juggling explosive growth with responsible lending is a tricky balance.

· Buy now, pay later, also known as point-of-sale financing, has been surging as consumers shift their spending online.
· Fintechs like Affirm, Afterpay, and Klarna have amassed several million customers. They're now looking to expand beyond their installment-lending roots.
· Affirm is exploring more financial products with...