If you're letting people enter in their credit card information at any point for any reason you MUST get an SSL certificate and force people to use a TLS secured connection. Infosec 101
Admittedly that's all happening with javascript passthrough to stripe's API using https.
What's to stop a malicious MITM (using an iframe, say) from fucking with Stripe's javascript during runtime if you're not using HTTPS yourself?
EDIT: turns out that Stripe themselves recommend SSL[1] for the same reasons we both guessed:
Do I need to use SSL/TLS on my payment pages?
Yes, for a couple of reasons:
* It's more secure. In particular, it significantly reduces your risk of being exposed to a man-in-the-middle attack.
* Users correctly feel more comfortable sharing their payment information on pages visibly served over SSL. Your conversion rate is likely to be higher if your pages are served over SSL/TLS, too.
1 https://stripe.com/help/ssl