关于off-site面试

I think you should briefly address why you cannot come to campus for interview. If you're out of the country, the reason would be obvious. If you're in the US already, it's important to show the school that you're genuinely interested in the program; you just couldn't come for reason x y or z. just my 0.02

非常感谢, 虽然我在国外, 但是去美国很方便, 所以我得说明理由.

I think schools will understand that we may not be able to come for campus interview for various reasons. For one thing, if you're invited to a school that's far away, you have to take at least 2 days off work. Those days really add up if you apply to several schools. (I personally am being selective with which schools I am interviewing on-site and which ones I'll opt for alum interviews). As long as your reasons are justified and you show your interest in the program, you should be all right.

Good luck!