Early on I thought I was clever and had no problem with the idea of receiving cards in a typical trading card box (or two) inside a USPS Flat Rate bubble envelope. The shipping cost for that is less than $8. But in the last few months, I have received two sets from sellers on eBay who have shipped cards in a trading card box inside the Flat Rate bubble envelope. In both cases, it was apparent that flat rate envelope went through the rigors of shipping, and the envelope did not provide any protection. The trading card boxes hit with some force on the corners and the cards inside the box suffered significant corner dings on the corresponding corner.
There are ways to remedy the potential problems associated with the Flat Rate bubble envelopes. If all the cards were sleeved, I think they would likely survive any drops. Or, if the cards were packaged with some force-absorbing materials on the ends (jam pads, 1/2 inch of cardboard, etc.), then a drop on the corner would be absorbed by those materials and not any cards.
But as a result of these recent problems, I won't accept a shipment in a trading card box inside a Flat Rate bubble envelope unless the shipper confirms they will take protective measures.
Put me in Coach, I'm ready to play. Today.
My eBay page
My BuySportsCards.com page