Weighted products refer to the net weight of the glitter. I always err to the plus side when weighing so there is never going to be less than the amount stated and is usually more. Packaging needs to then be added as postage is based on weight. If you wonder why your carefully totalled order isn't showing at the shipping weight that you expect this extra packaging is the reason.
Some products are light weight but are too big to be posted at letter rates - these are in the equipment area. One syringe needs to be sent as a AUD$7.45 parcel (within Australia) so these items are best purchased with a larger order to make the most of the package rate.