@Jeni-31 

Consider having a 3rd party inventory management app as the connector to integrate QBO and Shopify. You may start exploring from these options.

https://go.tradegecko.com/register?code=fiat-lux

https://www.zaperp.com/ref/25/

You may utilize the mobile sales feature as POS. But should you need more features in a POS system, you may explore this one.

https://hikeup.com/quickbooks?afmc=bk