When purchasing a membership through the online credit card page please remember that all payments have to be handled manually. These payments are usually processed within 24 hours, but can take up to 48 hours.
Note that a payment through postal mail will take longer than this because of the time it takes to receive the check or money order.
If you create a trial account prior to paying, the payment will be processed more easily and quickly, which benefits both you and everyone who paid after you. To create a trial account log in as guest, and at the command prompt type /register. After that just follow the instructions.
If you already have one or more accounts (active or not) whose details do not match the data you entered on the credit card site, this can cause a delay in processing the payment. Again, the best course of action is to set up a trial account before paying, or contacting an online admin to change the details of your old account.