Are you using hosted buttons or the standard buttons to create this page? Hosted buttons won't allow you to update the price... it is one of the advantages and drawbacks of that system. If you want to use radio button options like this then you will have to use the standard buttons and not the hosted to create the page.
The cancel button will appear on the next page and allow people to change their mind and go back. They can always click the browser back button. Since this is paypal's page, there is no way to update it.