Which of the Ecwid-supported payment methods offer a mobile-friendly checkout interface. I've tried the Google Checkout with my mobile Ecwid storefront and the checkout pages are definitely not mobile-friendly.
PayPal actually offers a mobile ready page if you proceed to it on a mobile device. Unfortunately, we cannot tell how exactly PayPal detects the mobile device and whether or not it will display the mobile versions for some particular device - please, contact PayPal support on that.
Eugene Efimochkin, Team