If I request that you send a fixture back to me for service work, here is my recommended packing procedure.
For this example, I am using an L4 fixture. The process is similar for the L2 and L8 fixtures. The intent is mainly to protect the potentiometer holders on the ends of each fixture, but also to protect the box from getting damaged by the heat sink. If the heat sink is not adequately wrapped (and also packed rather snugly into a box) then it can rip through the box in shipping.
First, wrap the fixture in some cardboard. I recommend getting this from USPS, because their Priority Mail boxes are free
Note: leave the cord(s) loose at this stage
If you are sending both fixtures, repeat for the other side
Sandwich the fixtures together. In this examples, the tabs from the small flat rate box make for a convenient way of routing and protecting the cables.
Note: Try to route and secure the cables such that they are not rubbing up against the ends of the heat sink
Protecting the potentiometer from impact damage is important.
For this example, I took 2 pieces of large-diameter bubble wrap and folded them twice, and wrapped around the end of the fixture, then taped it to the box
Tape the fixtures together also
The next step is to protect the corners of the heat sinks. Again, I use bubble wrap, but you could use foam or extra cardboard
If I have requested that you send the power supplies back as well, wrap the cords around the power supply & rubber band them, then wrap in bubble wrap. Alternatively, you can put them inside a small flat-rate USPS box
Shipping: Once the items are prepared as shown above, you don't need an oversized box for shipping. They can be squeezed into whatever box you might have available.
if the box you are using has a lot of extra space, you will need to fill these voids to make sure the items don't get slammed around in shipping.
You can use just about anything (newspaper, junk mail, bubble wrap, packing peanuts, etc) but the main rule is to pack it rather tightly. Peanuts will break up and move around, so if you use peanuts, use a lot of them. Paper/newspaper needs to be crumpled up tightly to make sure it doesn't get compacted more in shipping.
As always, reach out to me if you have any questions!