I have a problem that too many of the orders coming in don't get completed and remain in checkout. It appears as though it doesn't even try to send to paypal.
I don't think that the issue is that the form is failing validation because of 2 reasons:
This issue has been with me for quite some time, and I would LOVE to get rid of it...
In checkout is a stage when someone adds something to the cart but does not go through the checkout process. It has nothing to do with the way your site functions. Just the user didn't want to checkout.
This problem (most likely) is Shopping Cart Abandonment (Google it).
UC Followup is a module that helps users push through "Adondoned Carts". http://www.drupalmodules.com/module/ubercart-followup
If you do not mind to alter the core code there is a solution http://www.ubercart.org/forum/support/6399/many_incomplete_orders_in_checkout_status_what_could_it_be