If it's just a .pz3 file without anything else, you can just stick it in a folder and zip it up.  There is no specific directory for .pz3 files.

Now on the other hand if you were including the follwoing type of files, it would be a different story:

.cr2 / .pz2 / .pp2 / .obj / .mt5  (etc. etc.)

In those cases you would put them into an empty runtime and zip up the runtime folder.

Runtime / Geometries / FolderWithYourName / FilesInHere
Runtime / Textures / FolderWithYourName / FilesInHere
Runtime / Libraries / Character / Folder with files in it
Runtime / Libraries / Poses / Folder with files in it
Runtime / Libraries / Props / Folder with files in it

And don't forget to include a Readme file (named something other than "Readme.txt") that has your email address in it. Include if you allow the item to be used for commercial use.  And also whether or not you allow redistribution of your files.

"It is good to see ourselves as others see us. Try as we may, we are never
able to know ourselves fully as we are, especially the evil side of us.
This we can do only if we are not angry with our critics but will take in good
heart whatever they might have to say." - Ghandi