I actually explained the procedure that worked for me here on this thread:
https://bitcointalk.org/index.php?topic=5201488.msg53180342#msg53180342

A few clarifications:
1. typically, it works only for what they call USD order (means currently non-existing paypal and bank wire).
2. You need to establish what is your credit value. To do that simply say that you are putting in the order and need to know your credit. Just put the order in the cart, but don't finalize.
3. once you received the message about the value of your credit (or already know it), proceed to point#4.
4. Put in the USD order, finalize it, put the choice as bank wire. Bank wire has 48 hr leeway before it expires.
5. receive the order number, send email with this order number saying that you want your credit to be applied to that finalized order as a reply to the SAME rep who told you about your credit value and do it during 9am-5pm Beijing time. In my case they applied credit to order within 2 hr of sending the final email.
6. Hopefully, your credit fully covers miner's cost, then you do not need to pay anything and, hence, no need to sent any wires. That was my situation.
7. What happened to phil and others might not happen to you because I was specifically told: no refund for you, only credit and only toward new USD orders.
8. Obviously, if your credit does not fully cover the shipment, then you would have to follow instructions for the wire of the difference.