Because X.99 products are pre tax, so you really have to pay X.99 + X.99 * 0.0Y

This made me wonder about the origins of the 0.99 price. I had always thought it was largely to avoid theft, by ensuring the worker had to open the till for change on every transaction. Which makes sense if there's no sales tax, but if there is sales tax then you might as well round everything to the dollar.

Possibly sales tax wasn't a thing when this became a common practice.

Makes the argument for the psychological effect of 0.99 vs 1.00 much stronger these days though I suppose.

In the US, sure, but most of the world display prices inclusive of tax. (at least from my experience in EU and Asia)

Yes, we should totally revamp our coins to suit the needs of retail marketing.