Sorry, still having problems.
As mentioned before I have Store Pickup and zipship installed as my shipping modules.
By default zencart will select zipship on checkout. If I change to Store Pickup and try to continue it refreshes the page, gives me the "Your order's details have changed. Please review the current values and re-submit." warning, but zipship is still selected.
Just discovered another problem.
If zipship is unable to find a rate against the postcode then it displays 'Sorry. Delivery is not available to selected address.' and cannot be selected, and pressing the continue button basically doesn't work.
with checkout_one it does this properly in the shipping selection boxy, however it displays an old rate (plucked from where?) in the summary at the bottom of the page. And if the user pressess continue a couple of times it will let them through.
@royaldave, is this the zipship shipping module ... last updated in 2007?
I think you need to put in an address with a zipcode that doesn't have a rate against it, and try and force a checkout, and hopefully you'll see what I am seeing.