How to ensure that your customers never, ever... ever want to sign up for Prime... ever:
Step 1.
Prompt user to sign up for Amazon Prime while processing their order.
Step 2.
Customer clicks "No Thanks" (because customer was previously signed up for Prime and decided they didn't want to pay for it - so of course you should ask again and again... and again)
Step 3.
Display final checkout page to customer and automatically select the paid shipping option (okay fine, that's fairly normal)
Step 4.
Display popup window to customer asking them to sign up for Amazon Prime again (did I mention again?), only this time position the Sign up link in the exact same position that that the radio button for "Free Super Saver Delivery" is displayed (because you know that's the button they're probably going to select). Better yet, you should time the display of this popup so that by the time they've moved the mouse cursor into place, customer is clicking on said radio button at the very moment the popup appears.
Step 5.
By no means should you ask for confirmation before signing the customer up, when they do mistakenly click.
Frankly, don't know whether it was just exceptionally poor, or whether it was nefarious UX design to get people on Prime, but it's a really, really good way to annoy your customers. Or at least this customer.