Well, your comment is certainly correct.
Although I don't work for PM, they do charge for shipping on some orders and not on others as you state and as most other online vendors do also.
Some things ship to the US for free without a purchase minimum, while others can qualify for free worldwide shipping...
They still don't charge for standard shipping in the USA if you spend $50.00
But, instead of me quoting shipping policies, I'll post the link and you can just review them ;-)
http://www.penguinmagic.com/about/shipping
Best wishes...