This is an interesting idea and one that has a great deal of potential. When I was selling frags on a regular basis I used UPS almost exclusively. The rates were fair and I only had one incident that I can recall where there was an issue. I have an account with UPS and was offered a few different options with billing and discounted rates. No matter what service you decide on you are going to be faced with substantial shipping cost simply due to the nature of the goods being shipped.
I managed to get the packaging down to a science and was able to prepare a shipment in a matter of minutes. Keep in mind though that each vendor is going to have their own shipping issues. Since this is not going to be a single "store front" each vendor will need an account to ship with. Heat and cool packs will need to be purchased and on hand, along with foam for lining the boxes.
There are a lot of logistics that need to be worked out and without knowing that information I can't offer too many suggestions. I will be more than happy to share with you my shipping practices, what worked best and what I modified over time.