The checklist on the packing slip seems to indicate that I need to do something for each item: http://i.imgur.com/yyJXf.png

The first item is "ALL items are covered with the correct FBA stickers". So actually that is not necessary, and if I just buy a huge pack of candies and ship it to you with only this single packing slip on the big box containing many candies, then you can still sort things out?

If you are subscribed to the labeling service, absolutely. If not, the fulfillment center may do it as a courtesy. The most important issue is what the FC expects from your shipment. Any unplanned or unexpected prep activity will cause your items to be sidelined in order to continue the inbound process for all of the other shipments. If a massive shipment comes in that isn't labeled/labeled correctly the FC may elect to ship it back due to things like space considerations or man power needs to handle the prep.

Sorry for the delay in replying. Feel free to pm if you have any other questions.