Replying to Cancelled Orders..

So I ordered an item from an online store and received an email 2 days later that my item is not available in that size and my order will be cancelled. This is the second time this has happened. I won't mention the first store that did this (they were super helpful and gave me a discount because of it), but the second one might get a call out depending on how they respond through email.

How is this even possible? I'm not a web developer but I imagine there must be some type of automated inventory system available.. Now I have to wait for my refund, and I won't receive it for a week most likely. Basically gave them my money to hold on to for a week.

The item is still shown as available on their store in that size..

TL;DR My order was cancelled twice because companies can't figure out their inventories.

/rant