I did a small English to Korean translation for th...

I did a small English to Korean translation for them 90 days ago, and haven't been paid. I asked them for payment and they said there was actually a small link on the second page of the PO in the email where I had to login to get paid. They received my invoice with proper PO number and format but refuse to pay because it was not entered on the site for which I don't even know password. Humans should work like humans. This seems to be just an excuse to pay late or not to pay.