Sent my last two racks this way, with a warneke box, a small flat rate box and a padded envelope.
Step 1- secure warneke box and place in sfrb
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Step 2 - modify sfrb, cut/tear the box on the sides so the sides fold in like this:
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Step 3 - split the securing tab to fit in the now smaller slot.
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*you can tear off the extra tab
Step 4 - use the tape on the sfrb to keep the box closed.
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Step - 6 (optional) place your new sfrb into a padded envelope and tape the sides so that the padding is tight around the box.
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Ready to ship:
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I haven’t been able to test it but I imagine that you can wrap another padded envelope or 2, or you can also fit some bubble wrap in the padded envelope.
Frankly the modified box should be enough on its own, but their isn’t much of a cost difference between shipping in a sfrb vs a padded envelope. Also not sure if the USPS would accept that as a sfrb anymore.