The Merchbees Shopify app will send you or your vendor a notification when certain goods in stock are below a specified threshold. You are to put the stock level manually, and the app will count the quantity of sold and remaining products automatically. 

For high efficiency, the Merchbees app can send you a notification via email or Slack. You can also customize the email template and receive messages in your native language. You can create a schedule for getting these notifications or receive such alerts immediately after the inventory level goes below a specified threshold. For convenience, you can export and import your data in the CSV format.

Also, you can create a list of multiple email/Slack recipients to share each low-stock alert.

There are three pricing plans available. The free version offers inventory tracking of up to 100 product variants and the possibility to receive one scheduled alert. The professional plan that costs $4.99 per month (or $3.99 if you pay for the whole year) proposes up to 8,000 variants and unlimited scheduled and real-time alerts. The enterprise plan costs $8.99 per month (or $7.49 when paid once a year): it creates alerts on up to 100,000 variants, offers the ability to set maximum stock level, Slack notifications, and data import in the CSV format.