Immediately after checkout you will see two transactions on your credit card account. One is your first monthly installment and the second is the authorization, which is a required part of paying in installments. Only the first installment is a charge.
The authorization is NOT a charge and will not be due on your credit card bill. This is why you see the transaction pending in your account, but then it disappears. It is not a charge, so once the hold expires it is removed from your card.