One of my backers who's also on my Discord just posted up a screenshot of an email he got today. It claims to be from ShipStation (which is what I use for my shipping), but is asking for payment before his package will be delivered, and of course providing a convenient link.

His package was delivered by USPS, no charges, yesterday.

Here's a copy of the email, pasted together from screenshots, with the backer's name obfuscated:

Needless to say, don't fall for it. Especially not in the USA, where all shipping is prepaid, no duties, no taxes.