My order was declined, but I just checked with my bank or credit card issuer, and they say I was charged for the declined orders. Why is that?

In this instance, the charges you see are considered ‘PENDING’.  They do reduce your available balance; however, these charges will drop off from your account within 3-5 days, depending on your bank’s financial policy.  

At the time you attempted your purchase, your bank or credit card issuer places a hold, or pending charge, on the amount of the intended purchase against your card.

Transactions are typically finalized in the early evening when the card processor sends final confirmations for all of the day's transactions to the banks as explained above.

This is a general practice across industries for how online transactions are handled.