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I've been shipping a bunch of stuff lately, all with the flatrate boxes, and some times it's awesome (ship a heavy-ass iron turbo manifold for $13? Perfect!), but some times it's a tiny bit cheaper to ship regular Priority if it's something light (would have saved myself $0.53 if I used Priority instead of the flatrate box).  So yes, I am being sarcastic about being "careful" with flatrate boxes. But you knew that. Just know they're usually cheaper for the heavy stuff - lighter boxes will probably be more "expensive." 
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