It's likely you’re seeing a temporary payment pre-authorization, not a second charge. PayPal may pre-authorize a payment on your card when you enter your payment details. Once you complete your order on AllDayShirts.com, the actual payment is processed.
If you start the checkout process again, this could result in another temporary pre-authorization, but rest assured, only one payment will be finalized once you click the "Complete Order" button and receive an order confirmation number. The initial pre-authorization will typically be released by your bank in a few days.