When you select a credit card as your payment method and place an order with an onramp, they may temporarily pre-authorize your card to reserve the appropriate amount of funds for your transaction. Unlike an actual transaction, a pre-authorization doesn't deduct funds from your account; instead, it holds those funds until your transaction is processed.
You'll see pre-authorizations listed as "PENDING" on your statement, and they aren't actual charges. Pre-authorizations serve to verify that your payment method is valid and functioning properly, and to ensure that you have sufficient funds to cover the transaction cost while other necessary processes, like verifying your identity, are completed.