I am paying my mother's Humana monthly premium. Why am I being charged an additional fee due to late enrollment in Part D?

Call. Medicare. They should have the info when Mom applied. Did she work passed 65 and getting benefits from her employer? There is something about having employer insurance and allowing to be late.

Why are you paying her bills? That is her responsibility. Her Medicare can be deducted from her SS. See an insurance broker to find best prices with next enrollnent. Her monthly payments could be less.
As far as the penalty, that depends upon how late beyond age 65 was the sign up compounded with any pre existing condition. If the sign up was within 3 months or so of age 65, per existing conditiobs do not count.

Here's information from the medicare.gov website: https://www.medicare.gov/basics/costs/medicare-costs/avoid-penalties. Hope this helps.

We are not Humana.
We are a Forum of Caregivers.
We have no secret access to insurance supplements and their ways.

You should take your mother's Humana Card and call the number on the back of it and ask the question.

Wishing you the best of luck.