What happens if you send bitcoin after online vendor times out transaction?

First of all forgive me if this is a dumb question, I am a complete newbie. I am learning bitcoin and bought it, put it in my nano s wallet, and finally wanted to see how it worked to spend some. We happened to have an iRobot vacuum cleaner go belly up on us, so I thought I would buy a new vacuum using bitcoin to see how it works (dumb mistake, get a t-shirt for $8 for the first transaction, not a $500 vacuum). So I google companies that take bitcoin and Overstock.com does. I order a vacuum for $500. The transaction is timed with Overstock, you have to copy the bitcoin amount and the address into your wallet in about 20 seconds. Well it turns out that I sent the bitcoin about 1 second after the Overstock.com shopping cart timed out my transaction. So now my wallet is down $500, and there is no evidence of me paying anything to Overstock.com. I called their customer service department twice, and both times the customer service reps were totally clueless. They said no one uses bitcoin and nobody knows anything about it. So I have no idea what happened. Did Overstock.com still get the $500 into their wallet? If not where does the bitcoin go? Does it ever come back into my wallet? Thanks in advance for any help!