Unfortunately, due to the method PayPal requires for embedding a Bill Me Later banner, it does not appear possible to make that work within our cart. We have tested the embed code they provide and require, and while I've been able to get the banner code correctly inserted in our cart overlay, it doesn't actually work to display the banner there.
I suspect the trouble is that the banner embed code expects to render the banner upon page load, whereas our cart javascript adds our cart-overlay code to the page on-demand when a cart button is clicked after the initial page load, so we've got our cart JS asynchronously writing HTML into the page that would load PayPal's banner JS that would write more HTML... a real can of worms there. We've even tried tinkering with the embed code they provide to simplify how it works, which probably isn't kosher according to PayPal's terms for displaying the banner, but still couldn't get it to work anyway.
PayPal had contacted us to express interest in allowing Bill Me Later banners to be embedded in our cart, and we explained the issues above to them with our conclusion that the only way it could work is if they granted us special dispensation to embed the banner as a static image, rather than using their required embed code, but we haven't heard back from them on that proposal.
Meanwhile, it appears you could only use their code to embed the banners in your own site pages, rather than in the cart itself; they do recommend displaying the banner on every page of your site anyway, for persistent buyer awareness of the deferred payment option -- as they put it to us, "in order to achieve the lift in sales and conversion, the financing offer itself needs to be promoted starting from the homepage since we know that the ability to defer payments influences a customer’s buying behavior – and more importantly, their decision to make a purchase today."
