"but the ones shown on the subscription.php page were added using the ecart add to cart behavior."
well that would be the cause of the problem then.
you should not be using eCart to create additional add to cart behaviors, you should add the items to the power store catalog using the product admin
when you checkout, it will look in the products database to see if the product has unlimited inventory to ensure that more than are available can not be ordered.
the problem you are running into is that the products being ordered don't exist in the database, so the inventory check is failing.