How to best integrate an app with a shipping API?

Hi all !
I have a small shipping company in Europe.
I have a set of API’s, two of then create shippment order and list all orders of a client.

We want to develop a public app in Shopify, to develop a full experience to our customers (almost of then are web stores in Shopify).

I developed a front end using Shopify CLI and Polaris (in a development store) and everything looks fine.

By the way, I’m facing the following doubts:

  • What’s the best way to integrate my App Front End to my API’s ? (create shipping order / list all orders)
  • My API’s are using an API Key (provided by me at the moment we starts the partnership with the client). How can I store this API Key in Shopify App (I developed an user interface for my custormers setup this key, mainly because I’ll provide this App for multiples stores… but I’m not shore if this is the best way)?