I bought something online............?

and used my credit/debit card for the purchase. It was a little over a week ago I see the payment taken out of my account. Then I got an email saying that my package had been shipped. I checked my account and the transaction is gone and i have the $31 back in my account. I'm a bit confused. Is this something that happens often? Maybe what I saw was just an authorization? I hate to sit around forever having to always remember I have to keep $31 in my account just in case. Should I leave it alone thinking I wasn't actually charged or call the company. I asked my bank and they have no idea.