If the purchase didn’t complete then what you’re seeing is called a pre-authorization charge. This is a temporary hold on your account that takes place while the card processor verifies that the payment is allowed to happen. If payment fails at this stage (eg. you don’t have correctly zip code) then the hold may stay on your account for up to a day. The funds will be credited back into your account when the hold is reversed.
If the purchase did go through, but you weren’t redirected to your download page you can either check your email for the confirmation email, or request the email again from this page: https://itch.io/docs/buying/already-bought
If you still think you were mistakenly charged you should contact support and they can help with arranging a refund.
Hope that helps