[Shipping labels] Show shipping rate selection UI and purchase button after package is added

[rachelmcr]

After adding a package, we reveal the shipping service section and the disabled purchase button (previously hidden to reduce distractions and save space). We should also scroll to the selected package at this point.

The purchase button should be disabled by default, and enabled after a shipping rate is selected and all other requirements are met. ⚠ We need to double check the requirements for purchasing a shipping label; is having a selected shipping rate sufficient?

The purchase button should also include the total cost.

[dangermattic]

Thanks for reporting! 👍