I believe you are asking about Volumetric (Dimensional) Weight; please let us know if that is not the case.
For the private carriers (FedEx and DHL) they would calculate that as 5.5 kg of Volumetic Weight. They would use 5.5 kg as the weight of the package unless the actual weight was greater than that. You can find DHL's Volumetric Weight Calculator here: http://www.dhl.com/en/tools/volumetric_weight_express.html
For the Postal Service (USPS) they do not use Volumetic Weight and require dimensions only to determine if a package is within the size limit allowed for the destination country. So, for the USPS, a package measuring 30 x 30 x 30 cm would would weight whatever the scale says when the package is placed upon it.
Feel free to email us at Support@vykingship.com
if you have any other questions or would like clarification about anything above.