Last week Shopify Inc (SHOP), the online selling platform, announced excellent Q2 results with huge free cash flow (FCF) generation and high FCF margins. These imply that SHOP stock could still have a substantial upside, as much as 28% more. SHOP is trading at $74.71 in midday trading on Friday, Aug. 16, up substantially from when Shopify released its Q2 results on Aug. 7. The day before it closed at $54.22, so the stock is now up over $20 or 37.8% in the past week and a half. But it could still have more upside over the next year, based on an analysis of it strong free cash flow (FCF) and FCF…