Sendle has options to receive special shipping options when using carrier packaging. These generally help merchants get a 500g price when the parcel weighs a lot more.
For Sendle it requires the following satchels:
https://satchels.sendle.com/collections/frontpage/products/100-x-compostable-satchels
In this case as long as the items fit in the satchel the 500g weight can be selected when booking regardless of weight.
To use these, the merchant needs to use the drop down for “Package dimensions and Weight” where they can select the flat rate/unlimited package options.