Reorderly

A Shopify app that tells small stores what to reorder, how much, and when — from their own sales history.

Runs-out-in days and a suggested order quantity for every product, most urgent first.
Purchase orders per supplier as CSV or PDF, ready to send.
A weekly email of what will run out in the next two weeks.

Reorderly runs inside the Shopify admin. Install it from the Shopify App Store, or open it from your store's Apps menu if it's already installed.