ALWAYS check. many times (not always - so don't quote me) it's cheaper if you package it yourself. i just saved nearly $10 by packing myself on a shipment to europe. i found out by calling the post office. i found it impossible to navigate their website. they make it very easy to use their mailing boxes because it's actually easier for them, but it's hard to get other information from their site, so be sure to ask.
i've never been able to get the amount of fabric into those boxes that they say will fit. so poundage isn't even an issue. you're paying for pounds you won't use unless you ship gold bullion.