I certainly don't know about film, but the two manufacturing companies I've worked for it has indeed cost more to sell to small customers than large ones. The price of the product isn't determined at the end of the production line, it includes all the costs of storage and getting the product to the customer.
Just some random thoughts from my experience moving from small volume shipments to trailer load only volume.
Reduced staff on floor... full loads improve pounds shipped per hour numbers
more order volume, no increase in shipped volume...what is the benefit?
rather than truckload it goes LTL (less than a truck load) much higher cost
rather than full cases broken cases...more labor to pick orders, more inventory time, dock areas not used with efficiency
rather than just having case lot SKU's... may require additional SKU's for individual parts of the case, more SKU's more issues
case or partial case orders vs truckload may require a supermarket type picking area, not a good use of floor space
production planning may be effected. Length of product runs may be altered to insure inventory available to small customers
LTL type customers generally provide less lead time for production planning. Inventory costs money, may be borrowed money, or at least it is tying up money that could be used for other purposes (generating a better return).
Sorry about the ramble, but in many industries it is not the same cost to service big or little customers. I suspect film is no different.
On the online sales... many companies have agreements/contracts with their distributors to not undercut pricing. Yes, it generates additional profit
Mike