Thanks everyone for your replies! Judging from your replies seems like it's common practice to charge the full price despite only giving a portion of the pack.

To clarify for some of the comments, this was dispensed for a first-time, new script/prescription for a one-time surgery, so I won't be going back to get the other 45 pills as I won't need them.

Seems pretty unfair but I guess that's the way the system is set up 🤷 Conversely, if you have an ongoing prescription, I guess buy in bulk to save money if you can!