Hi Trip_Apple23,
Thank you for posting in the Apple Support Communities. It sounds like you're trying to purchase an album you've already purchased some songs from, correct? If so, we would love to help see if we can get the issue resolved for you. We know that when you pre-order an an album from iTunes Store, you are only charged for the current songs available and then the rest when the album becomes available as a whole, as mentioned in the below link.
Pre-order items from the iTunes Store on PC
It should work the same way as described with purchasing an album you have already purchased some songs for as well. If you'd like to be sure, we recommend reaching out to Apple Support directly by following the link below.
Contact Apple Support
Please let us know how it goes. We're always happy to help.
Cheers!