How do i estimate postage costs to send 35 pages?

That is too many pages for a regular envelope. For USPS needs to be a Large Envelope. See http://postcalc.usps.gov/ for calculations based on weight and zip codes.

If regular 20lb copy paper it is probably about 24 oz, about 1.5 pages per oz.

Divide page count by 3, round up to the next whole number, and double it = estimated ounces.