BTW the $1 store by me carries stryo box "coolers" that can be cut down to fit in a USPS shoe box. Worked great for fish and plant shipping 
You do the same thing I do. Any question the postal clerk asks me, the answer is "no". I have had enough disputes with postal clerks to last a lifetime. I am now using PayPal's label printing, postage paying service, and I don't need to even see a postal clerk again. My post office has a big mail box where I can just dump the Priority mail package and never see a single human face.I'm curious how everyone addresses the "liquid/ perishable" question when asked at the P.O.
If I tell them it's perishable does it get delivered faster? Do they want to see the items and raise a stink because it's aquatic plants and because of the liquid?
I've always just said "no liquids and not perishable" just to avoid complications.
I've been lucky. The postal clerks in this town don't ever ask unless I offer the information. But even if they did it wouldn't matter because they only want to know for the 'gossip-value' of the package.You do the same thing I do. Any question the postal clerk asks me, the answer is "no". I have had enough disputes with postal clerks to last a lifetime. I am now using PayPal's label printing, postage paying service, and I don't need to even see a postal clerk again. My post office has a big mail box where I can just dump the Priority mail package and never see a single human face.