How to fill checkout form fields with values from a previous custom form? Basically I have a few landing pages with forms with fields that are a subset of the checkout page form fields. I'd like for the user to submit the form and be redirected to the checkout page with the form values already filled.

I'm using Übercart and I think it's this function here that controls it:

function uc_checkout_pane_delivery($op, &amp;$arg1, $arg2) and the values of $arg1 needs the be set by the previous form. My question is how? What would be an appropriate method for populating the checkout form fields? Thanks.
I think you could get away with this by using hook_form_alter (checkout is still a form) and just setting #default_value on the required field. The previous form could save it in $_SESSION somewhere.