OK, here is how I got rid of the Appinventor - Username and use your desired name for Google Play URL.
I will try just text like this but if this is too confusing, I might try screenshots.
Download APK Studio
1. Compile your APK in Appinventor
2. Use APK Studio and click on "file/open APK" and select the APK you want to decompile
3. The app files are decompiled in a new folder when APK Studio stops decompiling at - com/google/youngandroid/runtime...
4. Open that new folder which should be created on your desktop which ends in -APKStudio
5. Right click on "Android Manifest.xml" and click on "Edit with Notepad++"
6. Highlight the part of the name you want changed, it can be the package name or Android name, it doesn't matter, then go to - "Search/Find in Files" and click on "Find All" and it will find all identical names in 2 folders
7. Go back to "Search/Find in Files" Click on "Replace with" and type in your new name, then click on "Replace in files" and your new name should replace all instances of the .xml and .yml folders
8. Open up the source file and get to the java files, then open up the first java file and repeat the above process for the package name. Make sure to wait about 5 seconds for Notepad++ to change all the names in all files.
9. Scroll down in that same java file to where your previous name is still listed in green writing and repeat the same process with Notepad++ again
10. Name your folders in the source file the same as your new name. For example: If your new name is - My.Free.App
then you will need to rename 3 folders in the source file to - My/Free/App
11. Repeat the same process above for the .smali files in the smali folder
12. Copy the .jar file and paste it into a new folder
13. Rename the .jar file in the new folder to .zip and extract the files in the same folder
14. Open the top folder and find the .class files and repeat the same process as the .java and .smali files and rename the first instance of your old name with Notepad++ "Find all" - "Replace all".
15. Now re-zip all the files and rename the .zip file back to .jar
16. Copy the .jar file you just re-zipped and replace the original .jar file in the other folder
17. Open up APK Studio again and click - "File/open directory", select the folder that you have been working on which ends in APKStudio and click "OK", then click "Build/Repackage"
18. Go back into the APKStudio folder and look for a new file called "dist"
19. Click on the "dist" file and your new APK with your desired package name should be in that file