Multiple locations with fulfillment app

[henny_ewhs]

Goodmorning,

We developed an application for our warehouse, where we store / pack and ship products for customers across different shops.

When customers only use 1 location there are no problems. The moment a customer wants to store an item accross multiple locations, it's impossible for the person to select our application as location. 

Stock sync is impossible and customers cannot buy the article.

What are we doing wrong, what setting within Shopify are we missing?