I no longer have my 13" MacBook Air, but do have the Transcend JetDrive Lite 128gb for sale. This is a 128gb SD card that is designed to be left in a MacBook Air 13" - it fits flush with the case so doesn't get caught on things.

I used it in order to instantly get 128gb more space, and kept my iTunes library on it. No other (easy) way of upgrading the space of a MacBook Air.

Bought for $150, sell for $75 inc postage.