You already know that you have been able to buy, deploy and manage add-ins from the Office 365 administration portal.
Well, this has been expanded to allow you to buy, deploy and manage Office 365 Partners apps too.

To use this new capability, logon to your Office 365 administration portal (https://admin.microsoft.com/) and access the SettingsIntegrated apps (formerly Add-ins)

There you will continue to be able to manage your add-ins but then also get third party O365 apps

The experience is also more integrated as the buy new add-ins or apps is now fully within the Office 365 administration portal