Stripe has an easy pay with card button. I want to hack it so I can pass a custom amount to it. (It defaults to 20)

The site currently under construction is http://pecosselfstorage.com/testpayments.html

I need the customer to be able to put in however much they want to pay. Whether this is in the pop-up after they click the button, or on the page next to the button doesn't matter much. I'd prefer it to be in the pop-up however.

Please don't just post links to Stripe's site, I've been through all of that and can't find any help there.

Any thoughts are welcome. Thanks!