I use these signs that landscape companies, other contractors, and politicians leave around town. Whenever I see one, if it's the right size, I snatch it up. They just end up in the trash otherwise.
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Then, I cut them to size and reinforce with toothpicks. I use toothpicks from the dollar store, 500 for 1usd. Slide 4 toothpicks into a corrugation. If they are not tight fitting, I snip a small piece from another toothpick and wedge it in between to create a tight fit. Reinforce this way at least every 4th corrugation (leaving up to 3 seed channels between corrugations). Put small paper wads into the ends of the channels to hold the beans in there. Label the channels.
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Then i put it in a 2in x 3in baggie I get for 1.79 per 150pieces at a local craft store in the beads section for making jewelry, and I put a 0.5g dessicant pouch I got on amazon 300 pouches for 9.99usd.
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Overall, my cost for making this package was less than 10 cents. It is a little time consuming, but it keeps the beans protected from the mail roller/crushers and humidity in transit.