They don't.

If both purchase attempts failed, the reservations will eventually time out and your bank statement will show the full amount on your account again.

are you sure it's posted charges or pending charges? not the same thing, either way you should take it up with steam support and/or your payment processor

I have purchased several software licenses via Steam and I have not had this problem. Most likely the account will be corrected if there was a failure. Have you checked the license and activations in your account?