So I have -4 for both of them, which results in -8 in total (same for left and right eye). At my optician, I have to choose the best package they have to make sure my sunglasses can be as light as possible (30% less thick). Now there is a problem, I wish to have my sunglasses polarised as well but they said it will make the glasses thicker, but I still can make it thinner again by buying an extra package for 45% thinner, so it may be as thin as the original glasses without the polarised option.

I don't know much about how physics behind how the glasses are made. But can someone help me to verify if my optician is being honest or it is just a trick to sell more products?