How to interpret "an ounce of ground coffee"
It's a weight.

It'd say fluid ounces if it meant fluid ounces, and fluid ounces are pretty much only used for liquid or liquid-ish things. Your package doesn't, and it's a solid, so seems pretty clear. Sometimes people are lazy about this in casual speech when it's clear it's a liquid, e.g. they'll say "a 20-ounce soda bottle", but even then the package will say fluid ounces, and in any case this isn't a liquid.

A kitchen scale is the best measurement method, and they're not that expensive and are useful for other things too. If you don't have one, I guess you could try to rig up a balance, and either use an existing 2.15 oz sachet or try to measure that much of something with a known density (e.g. water) by volume, but it's not going to be all that accurate. Measuring by volume is going to be hard for ground coffee, because the way it's ground affects the density a lot.