"Where is customs?"
"海關是哪裡？" should be corrected to "海關在哪裡？", not "海關在哪兒？" I ended up misreporting a wrong answer because I thought this was another question inappropriately disallowing "哪裡" in lieu of "哪兒".
The primary translation I'm seeing listed is "海关在哪儿？" with no 是. If you saw "海关是哪里", it was likely a user-reported but incorrect translation that was added accidentally. I agree that 哪里 should be accepted here, though, if it isn't already.
海关是哪里 is more akin to “what place is customs”. So “where is customs” can be considered an alternative translation if we actually understand the nuance, which you and I think is rather unlikely for beginners. The asymmetry between English and Chinese translation is so tricky.