The calculation is accurate in one sense, but I don't think it represents actual revenue loss for two reasons. Firstly, spending is non-uniform and peaks a lot

The calculation is accurate in one sense, but I don't think it represents actual revenue loss for two reasons. Firstly, spending is non-uniform and peaks a lot on Black Friday, near Christmas, etc. Second, assuming the site is down for only a few minutes (it's back now), a significant fraction of people will probably just try again later. I still imagine this is a significant loss.