I don't have a supplement business, but I own an office supply company. I use a drop shipper, which means I sell the product and submit my orders to this company to ship. They sell me the merchandise at wholesale prices, and do my billing for me. When the comsumer pays, I get my order back to resell. I'm sure it is a little different with supplements, but a drop shipper is good so you don't have to personally warehouse the stuff.
Hope that helps a little.