Here is a no-fuss technique on how to make a larger box for shipping out of two smaller ones. This will help keep your shipping costs under control since most shipping supply companies have minimum order quantities for large odd sized boxes and charge a lot to ship boxes to you.
Open the two small boxes up at the point where the inside flap is glued together.
Cut off the section from which the flap was removed.
Form and tape the boxes like you would any normal box except one side missing (1).
When both boxes were done, join them together where both sides are open to form one large box (2).
Use packaging tape on the inside of the box to hold them together.
Place the piece of cardboard cut off one of the boxes on the bottom of the fused box. Tape the piece on the part where the two boxes meet for more support (3).
Tape the seam of the box from the outside to secure the boxes together.
To use the box, put the item/s to be shipped inside making sure that they are properly packaged to prevent breakage. Close the flaps as you would a regular box and tape them shut.
Place the second piece of cardboard that was cut off originally on top seam of the box and use packaging tape to keep it together (4).
You may now take your package to your local shipping office after it was properly addressed and labeled.