Checkout.com has achieved impressive growth over the last year, increasing transaction volumes by 250% and picking up big-name clients. : Online payments start-up Checkout.com has raised $150 million in funding at a $5.5 billion valuation — nearly three times what it was worth when it last raised cash. Founded in 2012, the London-based company sells a software platform that makes it easier for businesses to process and take payments over the web. It competes with the likes of U.S. firm Stripe and Dutch processor Adyen.