用confirmation number 就能约成，到了再说。

而且 E-file里有一条：
You must contact the USCIS National Custer Service Center, if instructed to do so on your Confirmation Receipt Notice, at 1-800-375-5283 to Schedule an appointment with your local Application Support Center using your Confirmation Receipt notice or your offical receipt notice (I-797). The Application Support Center will collect a digital photograph, signature, and fingerprint from you at your appointment.
我觉得不能等待，等到他们寄给你指纹通知，太晚了。先约一下，如果不能弄，再回来等呗。