Shopify has its own custom URL structure as compared to other CMS based and coding e-commerce stores. You don’t get too much customization flexibility and it is hard to implement the existing URL structure. So you should be very careful while migrating your store to it.
As your question states, you are worried about the additional forward slash that appears in your previous URL. Which is probably not that serious issue as it may or may not appear and doesn’t affect the search rankings.
On the other hand if you completely change the URL structure and apply 301 permanent redirect to new one, it will effect your search presence. Because there are chances that new URL may not get that position quicker.
The only benefit of 301 redirect is you don’t loose already coming traffic to that indexed URL. By moving to new URL structure it will considered as fresh pages. This redirection also saves your backlinking efforts to existing pages.