We upgraded our magento from 1.6 to 2.0 and more recently to 2.1.7 but we've experienced numerous issues with checkout and our developers have still not found a solution. A big problem we have is when customers place an order multiple payments are taken from their account.
Another issue is that our magento has 3 stores which are for our US, Asia and UK sites and for that reason we've integrated stock inventory extension to manage the different stock levels for each region however, we are now getting the issue of orders not being assigned to the correct warehouse in the backend. Are there any solutions for this please??