Some people in my pms asking how I get a bunch of coins each time. First off, I have 10 different accounts each one with a different email, different shipping address from friends and family in the same zip code, and different cc numbers. For billing I use the same address as shipping as long as it's in the same zip code as your billing address, it goes through. Each Mint account has the address and credit card number in stored addresses and credit cards. The only other advice I can give is practice, know how to sign in and out, have the links for both coins handy, if you get "Gateway error" message don't give up, keep refreshing. I noticed it takes time from when you select the stored credit card at checkout till the card numbers show up, don't refresh just wait it out, it can take 10 seconds. Before clicking "I accept" and "place order" wait for the cc page to finish loading, you'll avoid lots of error messages. Good luck next time...
Eta: I never used incognito, don't think it helps. There were times (like 4-5 times) where a relative ordered the same coin to his address after I used his for shipping. Both coins arrived every time.