Shipping two singles in the US, but I've used this method for overseas too.
Scissors, coffee sleeve, Scotch tape, packaging tape, #10 envelope, 1 sheet of paper.
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Cut the coffee sleeve in half. Two staples in each half will keep the chips from moving.
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Trim the excess off the sleeves to reduce the rigidness of the final package. That's sometimes what hangs the USPS up (that it's rigid, not bendable). Tri-fold the sheet of paper and use scotch tape to secure the chips to the center. This avoids them moving around, and keeps them away from the edges of the USPS sorting machines.
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Paper goes inside a standard envelope. I put a piece of packaging tape over the delivery address so it doesn't get smeared or obliterated. I tape the envelope flap down to make sure it doesn't accidentally get opened. Then I put packaging tape along all four outer edges (overlapping front to back) to reinforce the envelope.
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Still "bendable"...
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Now, off to the Post Office for a non-machinable, first class stamp.
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70¢ later, two chips are on their way! Haven't lost one chip yet using this method (knock on wood). And FYI, Starbucks uses thinner coffee sleeves, which reduces the overall envelope thickness.