Non-machinable postage is "good" for up to 1oz which covers at least one chip, sometimes you can squeeze two. Otherwise you "need" an additional ounce stamp which I think is 15c.

The key is to make sure the envelope is a consistent thickness throughout aka they cannot tell for certain that there's an object/chip/coin within. Why? Technically, based on the USPS website, envelopes are only meant to be used for specific items. Chips are not one of them.

I used quotations above because as was mentioned earlier, PO to PO and teller to teller, their "requirements" vary to the extreme. I've received an envelope with two chips dropped in loose and the sender only paid normal postage (55c). I've also been told I needed to pay the +$3 fee for a sub 1oz envelope where it was nearly impossible to tell it wasn't just documents within. It's a crap shoot. My best advice is be polite and find a teller that will work with you and go to the PO when they're working.

Good luck!