Turns a near mint to mint card into poor condition.
It baffles me how some sellers are so cheap and lazy -- they stuff their trading cards in a soft card holder without a sleeve, and to make matters worse -- they don't protect their shipments with cardboard, as seen below:

As expected, that card was damaged during shipment. Here's a closeup scan of the damage:

That card also has micro scratches that you can't see in the scan. What's ironic is it wouldn't have cost much more to ship that card much more securely, as noted below:
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A card sleeve costs about $0.01.
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75pt toploaders which is much more solid than soft card holders, costs only $0.10 more.
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You can find cardboard for free.
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eBay's current USPS First-Class Mail rates for 1-8 oz packages is the same; in other words, the extra weight wouldn't increase the postage rates for that shipment.