What access do you need to fix my Shopify store?

The cleanest option is a Collaborator request (we send it, you approve it from Settings → Users and permissions) scoped to Themes and the relevant apps, no password sharing. A limited staff account also works. If you prefer, duplicate your theme and give us access to the copy only. We always work on a duplicate theme regardless, so your live storefront is never touched until you approve the fix.