Well, you said you have prescription drug coverage with Aetna. That IS Part D! And it is likely being taken out of your SS check, which is why they notified you that based on your income, you’ll pay a bit more. Remember when I wrote about Part D last fall, during open enrollment? That was the time to go to Medicare.gov, line up your prescriptions and see which plans are the least expensive given the drugs you currently take (or even if you don’t currently take ANY drugs).
So check your coverage by calling AETNA, and find out the cost plus the adjustment. I’m guessing they quoted the basic cost, and not the income-based adjustment!