Cannot select e-certs/credits when on the checkout page for a flight and in the payment area you click on e-credits. It bring up a list of them, but none are actually selectable.

If you start by going to your profile and certificates/credits link then you can view them and select a credit... however it only allows the selection of one credit even if they are combineable.
This forces you to have to call to book a flight with more than one e-credit.