POS Integration

I’m looking for a program to integrate a Squarespace ecomerce website with a brick-and-mortar Point of Sale application using the Order & Inventory APIs. I’m willing to give up Quickbooks POS but need a POS with similar robust features—Squarespace and Square will not suffice. I want to keep Squarespace, and the Shopify Buy Button is NOT ideal for a large complex website. I imagine there are similiar requests out there.

I’d be open to discuss a custom application…