 If you do not enroll when you are first eligible, you will incur a penalty of 1% of the average cost of Medicare Part D coverage for the number of months that you were eligible and didn't enroll. This penalty will be added to your monthly premium for as long as you have Part D coverage. We can show you how to avoid this penalty when you come in to talk with us. Keeping Original Medicare and adding Part D is one way to utilize your Medicare. The reason that most people add more insurance than just Part D is that there's no cap on the out-of-pocket expense that you will pay with Original Medicare.