Flatpay, which facilitates card payments for SMBs, has joined the ranks of European fintech unicorns — or startups valued at more than $1 billion — a milestone that has driven some of the region’s biggest exits. These include competitors like Adyen, a Dutch payment processing giant that remains far ahead in scale. However, Flatpay’s fresh funding could help it narrow the gap. Flatpay’s bet is that it can challenge larger players by charging small merchants a flat transaction rate to use its card terminals ...