If you can't try them on, don't buy them. Or bring them in and try them and if they don't fit, pay the restocking fee to your local shop.
Unless on continuous snow, you don't want plastic. If you are, you want plastic for any long period of time on non flat terrain and bring duct tape. Weekend warrior style, tenni's work fine in the PNW in summer. Guys even run up Ranier in them in June when crevasses are closed and the weather perfect... Not recommended btw but can be done.
Then again, It can be 70's in most NW alpine(5-6000 feet), but hit 8-9000 feet anywhere and it will be VERY cold sometimes with a temperature inversion even when crystal clear without a cloud in the sky. Generally not, but sometimes. Say going up Baker. Been in Shorts and no shirt on lower Colman glacier on Baker and wearing every stitch of clothing and wishing for far more near the top. Down Jacket, tights, shorts, wind pants, gloves, hat, long sleeve T, 100 power stretch fleece is what I had on.
BC is a different bird all together.