Off to the airport tomorrow morning, but realised that you’ve forgotten your dollars, euros or tögrög? This probably means you’ll get fleeced at their airport, where the buy/sell spread on foreign currency can be anything up to 25%. “Zero commission”? Hah.
However, it turns out that if you order online for in-person pickup, even from a branch situated in an airport, you get a much better rate. To give an example: on my recent trip to the US, the rates at the Travelex at London Heathrow were Buy: 1.42; Sell: 1.81 (see the spread!). However, by ordering the previous evening online for pickup at that same branch, I got a Buy rate of 1.57, saving me £12 on a £120 purchase, which is 10%. Not to be sneezed at.
You do need to order the previous day to get this (presumably, at least for common currencies, this is to stop people ordering online while at the airport itself) but, at least for Travelex, it can be any time up to around 11pm.
This works for UK residents, and may work for residents of other countries too – YMMV.
 Mongolia, in case you were wondering.