Another option Zenni offers (and most of the others do as well) is fixed tint on their glasses, in 10% (reflects the color of the tint, but doesn't actually affect your vision, nifty for 'rose tinted glasses' or the like you can wear indoors easilly) pu to 90% (great for prescription sunglasses).

Polychromatic + 1.67-index lenses would admitedly cost about $80 extra from Zenni per pair, but all of their glasses are polycarbonate as far as I've seen, and have extra scratch-resistance coatings.

And most of their glasses are available with matched-size-and-style clip-on tint carriers/sunglass attachments, as another option to the transitions.