App purchase on iPhone not recognized on Mac

Hello to the entire community.

I've developed an app available for iPhone and iPad, as well as for Mac, under the "designed for iPad" category. However, if someone buys it for iPhone or iPad, they have to pay a second time to get it on Mac. Even though it's exactly the same app. It's strange, and I can't find a solution.

Thanks in advance for your help.

[Re-Titled by Moderator]

Original Title: No twice paying for app