PayPal (NASDAQ: PYPL) is making its biggest acquisition ever, plunking down $2.2 billion for iZettle, a Stockholm-based in-store payments service provider. iZettle will provide a greater presence for PayPal in stores and in Europe, and enable it to compete better with Square (NYSE: SQ) for the business of small merchants.Square, meanwhile, will have to deal with a competitor that suddenly has a lot of cash at its disposal to fuel its growth.