You can still checkout within PayPal's safe environment. Pick the 'Pay with a Bank Account or Credit Card' option. If the option doesn't come up, you might used an email address that's already associated with a PayPal account, or your browser saved a previous PayPal login. Change your email on the top of the login screen, or -if you seem to get stuck in a loop of auto login - close the page, clear your browser history and cookies, then start the checkout process anew.

Once you entered your card details, make sure to uncheck 'Save info & create your PayPal account' on the bottom of the page.