Order total not discounted when "Coupon discount" pane is disabled.

This is because the javascript which updates the payment preview pane is only attached to the form when the coupon checkout pane is enabled. I will look into this when I have a chance, but a temporary workaround would be to enable the coupon checkout pane, and hide it with CSS.

Marking as a feature request, since most ubercart panes only function when they are enabled.

It doesn't make sense to me why this is marked a feature request. I am a maintainer for a site which supports credit card (through uc_payments), Google Checkout, and PayPal payments. In order to support using coupons for all payment sources, we display a coupon pane on the cart page. Because the coupon pane is displayed on the cart page we deemed it redundant to include it on the checkout page as well. However, if the user chooses to checkout using ubercart, the coupon discount is not included in their order.

It is possible, like you suggest above to just include the coupon pane and hide it with CSS, however, why should we have to do that? Why would coupons applied on the cart page not be included in the order (even though shown in the order summary pane) simply because we don't have that pane on the checkout page?

This seems more like bug report to me.

A longer term problem with the behavior reported in the original post is that if/when I uninstall the module in the future, (ex : I don't want to use the module anymore, or if/when the module becomes obsolete, or is not ported to Drupal 8 when I upgrade) then all my transactions with coupons will show an unpaid balance.

This is reason enough for me not to use the module, which is really too bad because it is otherwise a module with a lot of useful features, among them, the possibility to use reimbursement coupons to be redeemed over more than one transaction until the full amount of the coupon has been used.