I usually use one of the automatic postage calculators. Generally it asks if there is liquid and I select yes, and then it asks it it is over a certain amount, like a gallon or something, so I select no. It has never been a problem, never charged me more for shipping. There is also a question about 'live animals' which just so you know, although corals are actually classified as animals they aren't considered animals by the post office, so just select no.