To be fair to Zenni, most online eyeglass sites don't request vertical measurements as the PD is sufficient for the majority of single vision users and there isn't a single vertical measurement that can be applied from one user to other glasses. Ocular center height is very subjective & measured during the fitment of that frame to the user.
Optical center being the same height as geometric center is a safe choice for something only made to measure.
There are sites that take segment height for bifocals, but the intended frames need to be in hand for the user to measure.
