Hey bigfruits, just saw this post. If you haven't gotten answers yet, I just got some chips in Medium Flat Rate Boxes shipped via USPS recently. One was $13.50 and had just under 500 chips. The other box was $15.80 and had less chips, I seem to recall, but was from the west coast (don't know if that makes a difference or not).
A USPS Small Flat Rate Box can fit around 100 chips, for shipping around $7, if I recall correctly. If you have any of the 2 piece chip boxes for 100 chips, they fit perfectly in an SFRB but need some additional padding on the sides.