Browser caching leads to surprising behaviour when "skip payment" is selected

[geoknee]

If a file has already been downloaded, the browser will not bother requesting it again. If the user has selected "skip payment" we want there to be a 402 error.

We should look into whether it is possible to prevent this caching behaviour.

Or, if we use a different file each time that may be enough of a get around.

[lalexgap]

This only seems to happen in chrome but it's pretty easy to reproduce.