Chiming in here because I would also love an answer to this question. 

I think we found a work around. From the order page if you select more actions > view order status page, you're presented with an invoice page that has the payment link for the order. Not a perfect fix, not sure why shopify does not include the link ...

We too are experiencing this issue. But only with international customers. Has this feature been disabled on international shipments for a reason?