Battery of my mid-2012 Mac Air died (12% capacity) after 550 cycles, they usually just swap the batteries for free, but I bought my computer while living abroad, so no luck.

After some thoughts, I just bought replacement battery, two custom screwdrivers and changed the batter by myself (+ upgraded the hard drive) in about 15 minutes. Replacement battery cost around 150 USD here in EU and it is very easy to do.