Letters are typically very light and hence grams would be a better choice of unit for measuring the mass of a letter. Kilograms may be used for larger/heavier shipments/parcels, but as far as letters are concerned, grams will work just fine.
In SI units, the base unit for mass is the kilogram.
One kilogram is about 2.2 pounds. One gram is about the mass of a paperclip.
To avoid decimals, you should weigh an envelope in grams.
(If the mass of the envelope is 7 grams, you could say that it was .007 kilograms, but this seems hard to grasp compared to 7 grams.)
You would want to use grams to weigh an envelope or letter.
Let's look at it this way:
The SI unit for weight is the kilogram, which is equal to 1000 grams.
One kilogram is about 2.2 pounds. It also weighs the same as one litre of water. On the other hand, an empty envelope weighs 0.004 kg. That's 250 times lighter than the litre of water.
We can see why it's more practical to weigh super-light objects in grams than kilograms. In grams, the aforementioned envelope weighs 4 grams. That's a much nicer way to express its mass than 0.004 kg, because we get to avoid the many decimals in the numbers.
I'm not saying that it's always better to use grams, though. Sometimes, the situation might call for uniform measurements, and in that case, keeping everything to the same scale is a better option.