I have flown several award trips this year which were on short (but expensive) flights between SJO & BOG. BC tickets take double the miles of E, and I'm admittedly not usually willing to use twice the miles to sit up front for a couple of 1:15 flights. This route is an exception, but the general rule is that mileage-paid flights cost the passenger more than $-paid flights when booked in advance. Copa is the carrier I have been using, and I typically sit on row 5 - the first row of E. I often see open seats in BC, but they aren't available to me for upgrade because I'm flying on miles. It's a two-flight process, as I have to connect at PTY, and I had a particularly rough weekend (my best friend died on Sunday), so I was willing to spend the extra miles to move up last night. Not only did I sit on row 5 and watch several open BC seats go unused on both segments, I was further irritated by the fact that my request to pay the difference in miles was denied by both UA (the airline through which I booked the flights) and Copa. I can't figure out why my often dirt-cheap $-paid tickets still qualify me for an upgrade, yet my MP tickets don't. Even further puzzling to me is the lack of willingness to accept my offer to pay the additional miles to cover the upgrade myself. Had I waited to book the flight until the day before, they would have gladly taken the very same miles I offered and put me up front. With abundant availability, why in the world don't they want to help me clean out my account faster simply because I booked a couple of weeks earlier? I'm having great difficulty seeing the logic behind such a policy. It is the same policy on both airlines, so there must be something I am missing. Now that I am home, I'm glad that I still have the extra miles in my account to take another flight soon, but I really wanted some space last night. What am I missing?