Hello EmiB,
Thank you for using Apple Support Communities. It's my understanding that you're been charged for a few songs from a pre-ordered album and like to know when you will be charged the remaining amount. Also, you'd like to know why you have to pay for the songs when you have an Apple Music subscription. These are great questions! I'm happy to help.
When pre-ordering an album, you may be charged for songs released before the album. You will be charged the remaining price once the album is released.
See or cancel pre-orders for apps, movies, music, or books
With an Apple Music subscription, you can stream and download millions of songs as long as the subscription remains active. You don't need to buy songs or albums from the iTunes Store. However, you can still purchase music if you want to own a DRM-free version of the song. If you don't want to be charged, wait for the album to become available in the Apple Music library and access the songs at that time.
Add and download music from Apple Music or buy music from the iTunes Store
Have a wonderful day!