At this point, I think everyone knows that blue tape is a hobby necessity if you are sending out packages. However, there is another useful tool I've found for packaging cards, and it is cheap.
Again, I went to Dollar Tree for this hack. In the arts/crafts/school supplies section, I picked up a 20" x 30" foam board. In the tool section, I grabbed a box cutter. I own like 5 box cutters, but I can't seem to ever find them. Most folks use cardboard, but I've found that this foam board tends to be thicker, lighter and more rigid.
20" x 30" is a convenient size. Toploaders are 3" x 4". That means you can cut up the foam board into 50 toploader-sized sections. The board cuts really easily with Dollar Tree box-cutters. I first cut it into 10 strips. Then, cut each strip (or a couple at a time) into 5 pieces. If you have a coping saw laying around, I find it easiest to clamp all 10 strips together a cut them all at once.
Of course, you may want not want to cut the piece to 3" x 4". I usually drop 1/8" to 1/4" off each dimension so that they will fit comfortably with a card/toploader in a team bag:
This foam board was invaluable when it came to breaking my case of 2014 Allen & Ginter. I kept one sheet for odd-sized pieces. I shipped individual cards as shown above. I packed minis sideways into team bags, then sandwiched them between foam panels. I even use it to protect oversized boxloaders. It's always good to add a little thickness to packages without significantly increasing the weight. I also used pieces to fill out boxes. The thickness is perfect for filling all the gaps in a card box.
Additionally, you can cut these to 2.5"x4", and they make great monster box dividers. Throw an address label over the top edge for labeling.
Cardboard is good, but it can be heavy, flexible in one or more directions, and hard to find in large, convenient sheets. For $1, you can't beat a sheet of foam board.
It comes in black, too, if you're a badass.