Pleased to announce: One-Page Checkout for Zen Cart 1.5.4 and later!

This plugin's been developed from the ground-up, using the Zen Cart 1.5.5a checkout-handling as the basis. It's been validated on Zen Cart 1.5.4 and 1.5.5a with all the built-in shipping methods (plus ups and usps) and a couple of the more "complicated" (i.e. and PayPal Express Checkout) payment methods.

(Zen Cart v1.5.5a is the latest version of Zen Cart as of today's posting. No doubt there will be future versions. Consult the /docs/ documentation and/or discussions in this thread for further updates.)