I'm not totally sure what "Nope" is referring to — if you're seeing a Manage Subscription button, that's one of the buttons I mentioned in the post you quoted.
If you click Manage Subscription, you can then click Update Plan to apply the coupon under a field called "Add Promo Code". If you want the same plan, you can switch to another plan then switch back.
If you started a trial via the web, as you mentioned, you've already signed up via Stripe.com, which is the internet's largest payment processor. They store the same payment information across most sites you visit, which is why it's able to retrieve your previous card information.
If you need further clarification on any of this or need any help, please email me at
[email protected].