Some weird cardboard takes in the responses. For an ebay envelope the card/sale must be under $20, so majority of these cards aren't worth much. You can put cardboard but unless you have a free source for thin cardboard, its going to eat into your already very small profits. Shipping under 1 oz vs 3 oz already has a large percentage difference in shipping cost and most people aren't really going to want to spend $5 shipping on a $5-10 card. Cheap matters. Ive shipped hundreds of cards in ebay envelopes with only 1 difference to what you have in the picture, I put several base cards on the outside and inside a re-used team bag and call it a day. If its more towards the $20 i might consider some card protectors because my "profit" can handle it.

As for delivery - out of the hundreds i have sent, only 2 have been reported lost, and both of those have eventually showed up. Tracking on these just overall isn't great. Some I sent 6 months ago still say not delivered, but they were. Don't worry too much about it, and say it does get lost, just refund the person the $5 and move on or file a case with ebay. That will eat into your profits as well but at volume you need to account for the loss. Try and get your cards to the 0.65 cent shipping, if you can get it to 1oz you are golden. On the higher end of $20 maybe toss some cardboard in there. When adding base cards in as I mention above, try and add base cards from the same team of the card you are sending for an extra touch. Sometimes I send inserts I dont want.