The only way to get the credit according to their support is to pay with PayPal and then contact them with the order ID. They will then manually deduct the amount from the order.
That is not true, i am not saying you are lying, but their support is , it's likely the employee who responded to your ticket/email does not know shit, I have never paid using paypal on Bitmain , they never asked me to, and I still got the credit after sending them an email.
the first response I got was this
Dear xxxxxxxxxxxx,
Thank you for contacting us. I am forwarding your request to my colleague for assistance.
Best regards,
xxxxxx
Bitmain
The second one for her/his colleague was
Hello xxxxxxxxxxxx,
Thanks for your consultation!
We checked your order and found that the overpaid amount in your S9k order ID 0012019xxxxxxxx is 57USD, and it is left in your account as credit.
You can choose to use it in your new orders or refund, thanks.
Please let us know if you have additional questions or concerns.
Best regards,
xxxxxx
BITMAIN
If they tell you to use paypal, then send another request ticket and hope that someone who understand their job a little better will respond.