I’m looking for information on selling posters wholesale. How do you package them? Rolled in individual sleeves? Flat on cardboard with plastic cover? Do you charge extra for the flat? Where do you buy your sleeves? Cardboard and plastic?
Also, do you have a minimum purchase? Charge for shipping?
So many questions. Any and all thoughts are welcomed. Thanks!
Start more at the beginning.
Who are you selling the posters to?
How are they reselling them?
What kind of stock are you envisioning using (some of the nicer ones are too heavy to roll and some of the really cheap ones tear when taking out of the tube)
How are you getting them printed.
Sometimes your print vendor will supply all kitting (packaging and shipping services)
How big are they? If flat have you considered how you would package them rigidly enough to prevent damage in shipping? The bigger they are the harder that is.
Are you considering “archival” inks and papers? (often called giclee printing but take that with a grain of salt.) That determines where you buy your packaging and what kind of plastics you use (so does the inkset, some inks will stick to the insides of some plastic sleeves.
Of course you charge for the packaging 
And for the shipping
You are out to make money right???
My consulting fee will be in the mail. 
Uline is a great source for all things related to material shipping and handling.
Depending on the scope of the orders, I’d seriously check into printers that do fullfilments. But somehow, based on the basic-ness of this question, quantity probably isn’t a problem here.
Archival resources vary depending on the size of the prints. Archival dot com is a pretty good source. University Products is another. Both probably overkill as well.