I use to order a lot...I mean a lot...from many sources.
It seems I don't need to order much anymore, so my advice may be a bit dated...but here goes...
I placed many orders with Natchez, while their shipping fees were always higher than others, they had the best cumulative prices on the items I wanted to order, as well as having all the items I needed in stock, so, as long as the order was large enough, the high price of shipping and handling was spread out enough on the many items of an order to make it the best deal around.
BUT...I been disappointed a few times with the shipping/packing department. Poorly packed boxes where items would slosh around and get damaged...well nobody likes that.
I have ordered items from Midsouth, and opening boxes from them is memorable, it's like they are packing stuff that will be on a rough ride to the Moon. I've learned if I am ordering a random mix of items, Midsouth is the way to go, because saving a few pennies is never worth a poor packing job. If you are ordering one item or a "case" of an item, then it doesn't really matter who ships it out.