Reddit mentions of Scotch Transparent Tape, Versatile, Clear Finish, Engineered for Office and Home Use, 3/4 x 850 Inches, 4 Dispensered Rolls (4814)

First class mail package/thick envelope works absolutely fine with 4x8 bubble mailers. It costs about $2.67 if you purchase the postage through eBay, and you get the added bonus of free tracking and delivery confirmation. When you're shipping more valuable cards, you might want to spring for insurance (pay out of your own pocket if the buyer didn't request it specifically). It's pretty cheap and is a good way to play it safe.

 
Here's the method I've used for shipping hundreds of cards on eBay, with absolutely no problems whatsoever. You'll need the following supplies:

 
#000 4x8 Bubble Mailer
Penny sleeve
Toploader
Team bag
Scotch tape
Packing tape

 

1. Place the card you want to ship upside down in the penny sleeve, then fold the excess bit of plastic back up from the open end of the sleeve. This will keep the card from sliding out of the penny sleeve.
    
   
   
2. Slide the penny-sleeved card into the toploader, starting with the folded end of the penny sleeve. Be sure to keep the penny sleeve folded while you do this. It might be a bit tricky to get the card into the toploader, but it won't fall out of the penny sleeve once you do.
    
   
   
3. Tap the bottom of the toploader against a hard surface a couple times if need be to make sure the card is all the way inside and level. Then, tape a piece of scotch tape and tape up the open end of the toploader. Make sure to use a bit longer piece of tape than you need, and fold on end over itself to make a tab on one side of the toploader. This will ensure that the card won't fall out of the toploader during shipping while making it easier for your buyer to get the card out.
    
   
   
4. Place top toploader inside a team bag and seal it. In the event that your bubble mailer gets wet, this should ensure the card remains undamaged.
    
   
   
5. Print out the order invoice, fold it up, and place it inside the bubble mailer along with the toploader. Seal the bubble mailer, then place a piece of packing tape over the flap of the bubble mailer to ensure it stays shut.
    
   
   
6. Print out your postage. You can use a regular piece of printer paper if need be, but I'd definitely recommend going with an adhesive address label. Better yet, if you'll be shipping in volume I'd recommend getting a thermal printer (I've got a Dymo LabelWriter 4XL that's awesome), because then you don't have to worry about smudging, even if the address label gets damp.
    
   
   
7. Apply the postage to the outside of the bubble mailer. If you're using regular printer paper, be sure to tape all edges with packing tape to ensure it stays in place. Then place a piece of packing tape over the addresses. Again, this step is unnecessary if you invest in a thermal printer.
    
   
   Hope this information helps anyone looking for a worry-free way to ship cards!