I recently bought a SB-600 from Amazon and used borderlinx to ship. Amazon will indicated on their item description what is their shipping weight (which could be actual weight or volumetric weight) and it seems to be quite accurate. The actual product is very light but it indicated the shipping weight as 2 pounds (which I assume is volumetric weight) so I used this to determine my shipping cost. Got it during the GOLD BOX discount (extra 3%) and total damaged is about S$280+. Comes with 1 year international warranty.
Note only items ships direct from Amazon comes with free US shipping, other 'reseller' on Amazon will have shipping charges (usually) so take that into your calculation.
VPOSTUSA actually has a lower shipping charges (after factoring 15% discount for UOB Card Holder) by about $4-5 I think but I have used them before and they are quite slow. Sometimes take up to 1 week after items arrived at their depot before they are updated on your account. Then can take up to 2 weeks for delivery (though usually within a week).
So I tried borderlinx since is DHL end-to-end. I was quite impressed. Items shipped out from Amazon on 21 Nov, reach borderlinx on 22 Nov, was notified immediately, hit a slight hiccup for their routine credit check so on 24 Nov than I was able to pay and activate shipment, item left on the same day and reach me on the 27 Nov morning. Was not around at home, called DHL Singapore and re-arranged for shipment the same night.
I could never get this kind of service from VPOST and will probably have to pick it up from the post office myself.
BUT you will need to be a citibank holder to use the service and ALSO pay your items with citibank card or they may have the right to charge a fee for processing the delivery (as stated in their T&Cs, yes, I actually read them, and good thing coz I initially paid with other bank card but changed it before Amazon charge the card).
Overall experience: EXCELLENT.