How to sync inventory between POS system and new online store?

Hi - I am designing a new site for a client. They are migrating their online store to shopify but maintaining their current POS system for their Brick & mortar store to begin with.

They currently use a small independent POS software provider for their brick & mortar shop.

I need to be able to sync the inventory from their shop and the new shopify store.

Any ideas how I would be able to sync the 2 as their provider wants to charge £300 a month for the facility.

Thanks for your help.