We don't charge your card until your order ships. However, when you place an order, we immediately authorize your card to ensure that it can be charged when we ship your order. An authorization is a non-monetary transaction that merely checks with your bank to ensure that the card is valid and has sufficient funds. Again, no money is charged to the card until we ship your order.
However, at the discretion of your bank, an authorization can (and almost always will) put a hold on that amount of your credit. The duration of this hold is entirely up to your bank. If you are checking your balance via online banking, this authorization may show up as a "pending transaction" (or it may not show up at all). We do not have any control over the duration of these holds.
An authorization will only remain open for a set period of time that depends on your bank's policies. For debit cards, an authorization will usually expire in 1–5 days, while a credit card's hold can remain open for up to 30 days (again, the duration of these holds depends upon the card issuer's policy).
If an authorization expires before we ship your order, we must re-authorize the card before we can settle the charge. Since the average order on paizo.com takes anywhere from 2 to 11 business days to fulfill, some orders will necessarily be authorized twice. This is especially true for preorders, backorders and items coming from a distributor.
If, when you place your order, your card is declined because of insufficient funds, and you think this is in error, you should check your balance with your bank and/or contact Paizo's customer service before you attempt to place the order again. There are cases where we will receive a decline notice, yet your bank will accept the authorization and place a hold on your funds. In this instance, if you try to place the order again with the same card, it will probably fail again and put another hold on your account. If you keep resubmitting the order, you can quickly tie up your available credit on multiple authorizations for an order that cannot be completed; instead, you should verify payment information has been entered correctly and/or consider using an alternative payment method.
As a side note, orders for items that are fulfilled immediately (such as PDF and gift certificate purchases) will be authorized and settled immediately.
For more information about authorizations and authorization holds, check out the Wikipedia entry on authorization holds.