I just called Discover and asked them for the address that I can send irregular or remittance coupons to for payment. This is what they gave me:
PO Box 70176
Philadelphia, PA 19176
I also asked for the address of the CEO, I forgot to confirm the name, but a quick google search should give it to you. The address is:
PO Box 29037
Phoenix, AZ 85038
Hello. Suggested to read the cardmember agreement page. On it under "Payment Instructions" bullet points 4 and 6 (count the bullets because they are not numbered) tell you what they accept that's bullet 4. Bullet 6 tells you to mail it only to the address listed on your statement. Try and follow the instructions as written and see what happens. Hope this helps.