Is there a way to copy my store so that I can test an app? A sandbox store?
There’s not an easy way to make an exact copy of your store – you’d have to complete a full migration.
https://help.shopify.com/en/manual/shopify-admin/duplicate-store
However, depending on the app, you probably don’t need to make a full copy of your store.
You can definitely make a duplicate copy of your theme (so that if the app adds any coding, and then you remove the app, you can easily revert to the previous/duplicate copy of your theme).
https://help.shopify.com/en/manual/online-store/themes/managing-themes/duplicating-themes
As far as a test environment goes – if you sign up for the Shopify Partners program (https://www.shopify.com/ca/partners) then you can create a “development store” that you can use to play around and test things (separate from your actual store).
https://help.shopify.com/en/partners/dashboard/managing-stores/development-stores