If your package is too small or thin to qualify for a DCN, please make sure you get a receipt for the swap package (in case something goes wrong...)
I'm planning on either using a padded envelope or wrapping the finished item in bubble wrap before mailing it
I just used a personal letter size envelope. If your envelope is rigid or unevenly thick, you can get delivery confirmation. I also make sure that part of it is at least 3/4" thick. I also sell patterns and I always have to put a packing peanut in the envelope to meet the criteria for delivery confirmation. I've forgotten before and they charged the recipient for priority.
(not that she got priority handling.)