Improved payment page

Based on past feedback, we have made a few changes to our payment page to reduce potential confusion and make it more straightforward and informative.

Among the changes, we have made the price and purchased duration more visible, changed the language of our e-check warning to make it more explicit, and included a recap of your expiry date and what it would become if you made a payment with the currently selected options. We have also added an explanation (also on our billing page) of our expiry date calculation rules.