Currently, the default template for this private reply is:
Click below to view and pay for your order.
But if the customer is not yet confirmed, they need to click on the link to actually claim the item if they are not a fully confirmed customer who has confirmed on the page or group they are commenting on.
The item may actually have gone to waitlist as others purchased it, since they just had a temporary cart.
Suggested change to the template:
Click below to confirm your account, claim your items and pay for your order.