Google Checkout (the service used by merchants to accept Google Wallet payments on the Web) works as a "middleman" payment processor like PayPal, not as an ACH-style payment gateway -- i.e., the buyer pays the payment processor, which takes a cut as a transaction fee and then credits the remainder to your balance with that service, rather than an ACH direct payment where the buyer pays directly into your merchant account bank balance.
The only ACH-style payments supported by our service would use Authorize.Net or PayPal Payments Pro/Advanced/Payflow Pro (and the latter still allow for a PayPal account-based checkout option), and these are handled on a secure checkout page that we manage in common for all our merchants, which tends to throw a wrench into any solution that requires a checkout page on the merchant's site/domain or otherwise under the sole control of that merchant.
With both PayPal and Google Checkout, once the buyer clicks that checkout button in their cart, we hand off their cart details to the payment processor, where actual payment is arranged completely outside of our influence. When the payment processor completes their processing of the buyer's payment, they transmit the order details back to us in a completed payment notification (e.g. PayPal IPN), so we can process that order for you.
Our current, Flash-based Admin has become too complex and crufty to modify without breaking it in unpredictable ways, so any new features or functionality that would need new settings to manage them have had to wait for an all-new Admin that we can actually modify; therefore, completing that new Admin has been our penultimate Development priority (behind maintaining our current system, of course).
Adding support for PayPal Advanced and Payflow Pro was pretty much a matter of PayPal twisting our arm; we had little choice in the matter if we wanted to stay in PayPal's good graces, and the way we had to go about adding settings for them was, frankly, a rather clumsy stopgap that will finally be properly integrated in the new Admin.
HTML and CSS are generic terms covering HTML v.1-5/XHTML and CSS1/2/3/4 respectively. XHTML is not a successor to HTML4, which would be HTML5 (currently still in development); XHTML is a fork of HTML that also complies with the XML standard, intended for use in special cases where HTML also needs to be processed by XML software tools. Our new Admin code will not need to interoperate as XML, so I'm avoiding the unnecessary code bloat of XHTML by building it in HTML4 that will also be valid HTML5 (insofar as the latter standard has been developed so far).
Finally, bear in mind that E-junkie is deliberately positioned as a simple, basic, reliable solution for simple, basic needs. We cannot be everything to everyone, and trying to become that would turn us into the sort of sprawling, bug-ridden bloatware that nobody wants -- indeed, the very sort of thing we were created as an alternative to. There's a plethora of all-singing, all-dancing ecommerce platforms already competing out there, so we're not even trying to compete with them on their terms; we're going for the under-served demographic that finds such solutions to be daunting overkill, yet who don't want to be captive to a marketplace site like Etsy, eBay, or Amazon.
We are like a VW bug -- simple, basic, reliable and easy to maintain yourself; if your business has grown to the point that what you really need is more like a cargo truck, you'd do best to just get one of those, rather than expecting VW to trick out your bug to be more like the truck you actually need.