We'd love to use Shopify's integrated shipping. Unfortunately, they do not allow any integrations from the outside.
Your options in this case:
1) you can create labels outside our app, this is what we call "manual Prepaid Labels" but tracking will not work inside our app in this case.
2) you can connect your own contract to our Shippo or ShipStation integration (Shippo and ShipStation are basically platforms for creating labels) and use our automatic prepaid labels
3) you can use one of the already integrated contracts in Shippo or ShipStation (USPS, UPS, DHL Express, Hermes UK, Sendle, Parcelforce, etc). In this case, you do not need a carrier contract yourself. For details see Do I need my own carrier contract? for details.