If users identify themselves during Checkout without loging in, the system completes the purchase based on the already available profile information, including the address, making it impossible for the user to simulate other shipping options.
This occurs because the shipping simulation requires, without exception, for a new address to be inserted in order for it to work. And, to that end, the user would have needed to log in. User data will continue secure.
In repurchase scenarios, inform users that they need to sign in to simulate shipping on the product page.