Ladybugs are harvested from the wild in Sierra Nevada foothills where they migrate in spring
as the Sacramento and San Joaquin valleys warm up. There they rest and breed on the floor of pine forests in large numbers. Humans come along and scoop them up, removing them from their native habitat. They are then taken to coolers for storage until they are going to be shipped.
One they arrive and are released in someone yard they usually migrate before feeding or laying eggs, providing little or no control for your pests. Another concern is harvested ladybugs may be parasitized by a small wasp, Perilitus coccinellae. It develops as an internal parasite of lady beetles and kills them.
Harvesting ladybugs from the wild is not environmentally friendly.
If you want ladybugs the best thing to do is attract them in naturally with planting things like, dill, yarrow and other assorted flowering plants. If you want to do a release of a beneficial insects release laboratory reared lacewings. They will do an excellent job of feeding on plant pests such as aphids, mealybugs, scale and others.
As to why you ladybugs died, could be a few reason. Some pesticides can have residues for several months, or even other chemicals that may not be labeled as pesticide but may be toxic to them.
Also ladybugs being in containers in that close of an environment promotes disease issues. It is possible they were sick and the stress of shipment made it worse. Also if water was added to the container this could have promoted a disease problems. Paper towels carry chemicals that are known to kill caterpillars, maybe ladybugs too if exposed to them?
Was it to cold for them at night? Remember you are not release ladybugs from your neck of the woods. Are you seeing your native ladybugs out yet? Also are the ones you are seeing just the dead ones form the bottom of the container? There are always fatalities.
Hope this helps!!