Regard to what you put in, and what you take out, lets keep it fairly flexible...
I know I've got some packets where I've only got a few spare eg 3 or 4 melon or winter squash seeds, but that still might be enough for someone to try a new variety, so I'm going to put them in. At the other extreme I've got some unopened packets of flower seeds which I'm also going to put in - might be 100s in some of those, but people don't have to take them all if they're not thinking of growing that many... just take what you realistically think you're likely to grow.
I was about to say that large numbers of big seeds like beans might not be such a good idea (due to weight and size) but then thought if they were unusal ones like crimson flowered broad beans, or cherokee trail of tears beans, they might be a hit! So I wouldn't rule that out either. (Don't get excited anyone, I haven't got any of either of those to add...)
Only other thing I can think to add is keep the parcel to a size that can squeeze through letter boxes incase people are out when postie arrives!
HTH :)