I'm going to bump this year-old thread for a simple PSA regarding using one-click at Amazon.
I used it often, but never really took the effort into realizing exactly what it does. Basically, it creates a timed order - I think 90 minutes. All items ordered with 1-click get combined within that 90 minute period and THEN the order gets processed.
Well, if an item is in heavy demand, do not use 1-click ordering. Item availability is calculated at the time of processing, not using 1-click. If it sells out before the 1-click timer, your order will be processed, but backordered.
Just an FYI.