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What size coin bags do you need to fit $100 face in pennies inside them, and then ship, 12X20?

You probably don't want to ship them all in the same bag. The standard size coin shipment bag will work fine for up to 7500. Just use 2 of them and ship in the same box. They say they are rated for 50 pounds. They are 12 inches across the bottom and 18-23 inches long, depending on manufacturer. Be sure to use lots of tape on the box.. the PO loves those heavy boxes.

Look for thread on how to stiffen large flat rate box an use plenty of tape the PO uses heavy boxes for kick Ball

Here is the thread for the guide to penny shipping.
viewtopic.php?f=18&t=201