Drupal 6 Ubercart - cancel/remove payment on order
Rob, I'm afraid it may not be possible. In Ubercart, we handled shopping carts as pseudo order objects, tying cart items directly to a session. Once that cart has been checked out, as I recall, there's no way to run through checkout again with it. You may just need to contact the customers to resubmit the order or process their payment some other way (PayPal money request?).

Fwiw, this is why we made the decision in Commerce to manage all shopping carts as full order objects and use the order ID in the checkout URL. Much cleaner to support this type of behavior.