Plan-based payments/Recurring payments/Fixed cost payments.

This is a big omission in the system that I'm sure a lot of people want: The ability to take fixed payments, like "$50.00 to donate a cow" and also "$20.00 for monthly subscription". Stripe allows this quite well via "plans", which I actually implemented through the wp-stripe plugin recently. I'm not totally happy with _how_ I had to implement it, though.

What I did required passing parameters through the iframe url for the modal window using custom shortcodes to create different plan buttons, like "sign up for gold plan" and the like. Then each of those buttons passes parameters to the link or to the modal window iframe that specify which plan is being requested. I also (a hack) passed the display-only price, though in retrospect the price-of-each-plan should probably be retrieved from the stripe api via a curl call in the script, complete with all the page-load-slowdown that is likely to entail. Overall, it's not an elegant solution at the moment, though it allows use of plans, so it does what I needed it to.

I also found the use of the modal window troublesome overall, since mobile browsers totally stop working when in contact with the modal window.

@noeltock @mrjameshamilton I will create a branch so that I can show my work on this so far, but in the meantime do you guys have thoughts on ways to improve plan-based fixed-price and subscription type payments? I could especially use some brainstorming on how specifying that plan X is being requested should be most gracefully passed to the final payment form, as currently I have to do this as a url parameter to the iframe, specified by shortcodes for wp stripe.

Start by reviewing the existing shortcode and iframe/modal payment flow described in the issue. Determine how plan selection, fixed prices, and recurring subscriptions are currently passed to the final payment form. Done would require an agreed design and implementation for Stripe plans, pricing, and the reported mobile modal behavior.

Written by the indexing model from the issue text.