I am guessing they might be still debating whether they should extend the offer to you since they asked you for a campus visit though they knew the travel cost is so high. Not sure why they wanted to spend $2000 to ask you for a campus visit if you already confirmed to accept the offer. You may need to try to ask them send out the offer letter before you go and try to ask them if you can visit their campus later after you get the letter.
要我Confirm的时候,我还啥也没看到呢。 就是面完了问,Please confirm you will accept the offer, 我confirm 之后又发邮件说 please confirm you are still interested with us and will accept the offer, 我又confirm 了之后发邮件说 we decided to extend an offer to invite you to visit us in following weeks..