Grams and kilograms are measurement of weight. For a letter you would use grams. For a package you would use kilograms as it weight is more.
I would describe their weight in grams.
kilograms because grams are smaller than kilograms.
320 grams = 0.32 kilograms
If I weighed less than a kilogram I would of course express my weight in grams
The weight of a bicycle would best be measured in kilograms.
You would use Kilograms to weigh a cat.
The weight of a signle carrot would best be measured in grams.
The weight of a single potato would be measured in grams. A bag of potatoes would be measured in kilograms.