I found these in the same shop as the Silly Feet silicon muffin cases. Originally intended for making bouquet garni, the bags can also be used for making herbal teas. Or in my case coffee.
They come in packs of four and each bag weighs a miserly 2g. I put a tablespoon of cafetiere grade ground coffee in each bag, enough for that morning caffeine kick. When finished with the bags can be turned inside out and rinsed out. I enjoy a morning coffee when out on the hills so I'll just take a bag for each day I'm out, which is a much lighter option than a mug mate and a container of coffee.