For up to four chips I use a regular #6 or #10 envelope with the chip(s) wrapped in cardboard. I will use a "butterfly" stamp or, if I don't have one available, a regular first class stamp with an "additional ounce" stamp. I also indicate in RED both front and back that the envelope is "NON-MACHINEABLE". If the weight exceeds 1 ounce I add another "additional ounce" stamp. I will generally fold the envelope a couple of times to show that the envelope is not rigid (apparently one of the requirements) and make sure the fold lines show. Have not had any problems with this at all.

More chips, or more valuable chips, I will send the more expensive way with tracking.