Does Mac App Store provide an email support channel?
If you purchase an app from the Mac App Store, the purchase receipt you receive via email contains a “Report a Problem” link.  If Apple replies to your problem report via email, that email can contain a return email address for the particular Mac App Store problem you are reporting.

Each app developer is also required to have a support link for that app listed in the App Store.

For general support, Apple recommends using their Support App. See:
https://support.apple.com/contact

There are also Apple community support forums: https://discussions.apple.com/welcome 

and https://discussions.apple.com/community/app_store/using_app_store