If you've already got your store setup on another platform, like Statamify, Shopify, Craft Commerce etc then you don't really want to have to go through the pain of migrating everything by hand.
From the Simple Commerce side of things, we've tried to make things as easy as possible to get stuff migrated.
Although, some platforms, like Craft Commerce or Shopify don't have a really great way (first-party) way of exporting data, so you may find yourself needing to write some custom code to do that for you.
Over time we're hoping to write some scripts to help you export your data from popular alternatives into a format that Simple Commerce can understand. Once we've done that, we'll throw them up into a Gist and link them below.