[Exhibitman]

Per the USPS:

Dimension Minimum/Maximum
Height 6-1/8 inches (15.557 cm)/12 inches (30.480 cm)
Length 11-1/2 inches (29.210 cm)/15 inches (38.100 cm)
Thickness 1/4 inch (0.635 cm)/3/4 inch (1.905 cm)

As long as you fall within the min-max parameters on one of these three dimensions, your item is a large envelope. However: "Large envelopes that are rigid, nonrectangular, or not uniformly thick pay package prices." Hence the peanut solution.

So, a slabbed card in a bubble mailer should be just fine as a package rate shipment due to it being a rigid large envelope.

I haven't had any reclassification issues with outgoing mail because everything I sell goes out is at least 1/4" thick and rigid--either a slabbed card in proper wrappings or a raw card between two pieces of corrugated or equivalent, or one of those rigid cardboard mailers--so it gets the package rate. FWIW, I've found that eBay's mail label system is convenient and offers decent discounts from retail postage on packages. Plus it integrates the tracking data for us lazy people.

I would rather spend a tad more to ship with rigid packaging than use a packing peanut or other lumpy item to create sufficient bulk to qualify for the package designation because I have had a number of items arrive with new creases in them when the seller put the card in a card saver or top loader with nothing more next to a packing peanut and the nimrods at the USPS squashed the package during normal handling.