fence post setting regular quickcrete vs.fast setting
If the setting time is not an issue, you can certainly use normal quickcrete - or save even more money by purchasing cement and aggregate (sand/gravel/rock) and really making use of the mixer.

Standard concrete mixing practice is to mix the dry materials and then add water. Common error is to add too much water and make the resulting concrete weak, though fence post setting is not exactly a high stress job, normally.