I've heard that usually only UPS and Fedex charge duty taxes.

So if I were to say, order an 8 OZ starter kit ($34), using First Class Mail would I be charged with duties?

no, lower than 60$, you wont have to pay any duty... they simply drop it in the mail by canada post

I wrote a fairly lengthy post on this back in June. People often get confused by the various fees, and call it all "duty," but it is not. Here's a link to that thread:
Customs Processing Fee, GST, Duty, & Brokerage Fees

I think the "by the books" limit for no fees is $25, but the reality on the ground is around $50. It kind of depends on which mail processing/customs point your package enters Canada, and how heavy of a load of other packages there are on that particular day. The truth is that there isn't enough staff to clear each and every parcel. On the high volume days, the lower value packages are passed through, and customs takes their pound of flesh from higher cost items.

So you can't really count on your parcel making it through unscathed each and every time. If customs looks at it, you will be charged their $5 processing fee, and GST on the value of your order. If they don't, you get away scott free.

To be honest, since I live in Alberta, I don't know if PST gets charged as well. Perhaps somebody who has had a postal package processed by customs can fill in that blank spot in my knowledge.

I haven't done the math to figure this out. There is a point where GST plus processing fee would be more than the price of shipping another small order. The "starter kit" should slip under the radar. I'm sure the 16oz size, full package of cleanser, BP, and moisturizer are over the dollar value that makes customs take notice. If PST comes into play, that certainly could change the picture. Some other day I'll have to throw all those numbers through a spreadsheet get the answer.