The request for more robust Ubercart add to cart forms comes up fairly regularly. People generally have in mind the sort of form that allows customers to specify quantities for a set of products and add them to the cart all at once. The request came up so often that I put a short explanation on how to accomplish this through a custom form into an FAQ post on Ubercart.org.
This article will walk through a very basic example of how you can use the Ubercart function uc_cart_add_item() in a custom form submit handler to add multiple items to the cart at once. This article assumes a basic knowledge of Drupal's Forms API. Code examples are from Drupal 6, but they are mostly applicable to Drupal 5 as well.