I'm told I need activity on my United miles by the end of the month, or they'll all expire. I'm further told that the easiest and least expensive way to do this is by connecting from the Mileage Plus Mall to the iTunes store, or buy.com, or some other internet retailer.

The problem is that these retailers, even when I connect to them from the Mall, don't seem to understand that I want to pay in miles. I bought a song from iTunes, only to see my existing credit balance debited. Buy.com offers a variety of checkout methods - Google, PayPal, etc. - but not "Pay with your UAL miles."

What am I missing? What are the nuts and bolts of getting activity on my Mileage account? Thanks.

(And boy am I annoyed with UAL for wasting our time and making us jump through these hoops. )