Do you really want to tell the shipper how to ship it? You start making decisions and if something goes wrong it is on you.
I tell the seller "take whatever time you need to package and ship it safely. I'm in no hurry."
Customs process is a different story. They will have to fill out customs forms with a harmonized product code on it. What code they choose can affect what duties you pay. I'd do some research and suggest the proper code. If they choose one for defense electronics or something with special laws it can delay and cost you money.
Jerry