So maybe something like this senerio...
This Seller has 52 unique items to sell
Has a $45 FedEx overnight ship rate
A Free ship threshold of $135
Buyer 1 wants only 3 items and knows it isn't enough for the free shipping threshold but only wants these 3. Seller charges only one flat rate for all items purchased (I think after the first item is added to the cart from this seller there needs to be a "if" for more than one item ordered to not be charged shipping again provided they check a multi order check box or something like that just before clicking add to cart). This way they are only charged once for the multi order.
Buyer 2 adds 3 items to cart but it is under $135 (classifieds sends a pm to say something like "Just a little more and you have free shipping!" because it know the seller added a free shipping threshold)
Buyer 3 adds 2 items and knows he gets free shipping because it is over the sellers threshold and does not calculate shipping to this order.
---- on the seller listing he could choose a threshold amount or if he will have one at all:
so for example
Is this item included in a free shipping threshold?
Y / N
Threshold= [$000.00]
Adding Shipping to each item?
Options/
Yes each item
No only 1st item
Buyer side at listing:
a check box if this is an additional item = not to charge additional shipping
I'm probably missing something but I think you get the idea