Here's my initial shot at a multipage cart module (actually it's my second, but my first was a complete custom rewrite, doesn't use the panes system and is quite separate from ubercart).

Basically what I've done is used the form_alter hook to render the checkout form, but depending on the page you are on, only a subset of the panes are visible.
This way, all Javascript used in the panes still works, and hopefully the majority of panes won't even realise that they're in a multipage checkout.

UC Restrict Qty lets you add a very basic Restrict Qty. to a product in your Ubercart store to limit the quantity of that product in the shopping cart to 1. This is useful for single purchase products or for instances where it just doesn't make sense for a customer to purchase more than one and you want to prevent it from happening.

Due to the way products are added to the cart, customers can add multiple variations of the same product to their carts (i.e. different attribute/option combinations) but never more than one of any given variation.

This module adds 'Save for Later' functionality to a cart.

Optionally skip the review stage on the Ubercart checkout page. The store admin can set whether checkout review is required in the checkout settings.

The uc_addresses module adds support for one or more addresses in the user's profile. When users register, they must provide an address. Users can then add more addresses and edit or delete existing addresses. One address must be designated as the default address and cannot be deleted (but it can be edited).

The Ubercart order process is altered so that users select delivery and billing addresses from their collection of addresses rather than from previous orders. Any new addresses entered during the order process are automatically added to the user's list.

This module adds PayPal Buy Now or Subscription buttons to product pages (on the Add to Cart form). These buttons redirect the customer immediately to PayPal for payment, bypassing the standard Ubercart checkout process. Order information (including the billing/delivery address) is obtained from the PayPal IPN.

To enable a product to be sold with PayPal Buy Now or Subscriptions, add and configure the 'PayPal Buy Now' or 'PayPal Subscription' product feature. The product feature includes settings for the button style, as well as for the product or subscription.

This module works in conjunction with the payment checkout pane using the order total preview to present a "Free order" payment method to customers when their order total drops to $0.00 or less. By default, this method will be hidden, and server side verification of an order total will be used to prevent customers from gaming the system.

This patch for Ubercart beta 7 makes the creation of a Drupal user account for an anonymous customer optional.

From business point of view, shopkeepers basically want their clients to:

1. Find a product
2. Add the pruduct(s) to the cart
3. Proceed to checkout and pay

The checkout enhancements module provides a few tweaks to the checkout pages that I wasn't quite ready to drop into core so late in the 1.0 development cycle. These features are up for testing and feedback, and the enhancements as they are or modified versions may find themselves part of core in the future.

Currently included enhancements

• Removes the cancel button from the checkout form.
• Adds a throbber and disables the 'Submit Order' button to prevent multiple clicks.

There's a simple menu to toggle enhancements on or off at: