eBay is certainly not going to to extend the exemption for items to ALL large letters. The most they could possibly do would be to extend the low cost / low weight exemption, although they would first need to work out a way to counter the enormous number of people who would dishonestly declare their weights/dimensions to take advantage of this exemption.
Although it's a factor, price is not the only reason for this. As part of its contract with RM/PO, eBay will have agreed to a maximum average weight of parcel. This won't be as simple as the Royal Mail bands, so it won't be 2kg, or 5kg, or something like that.
It's more likely to be something like 1.177kg, a figure which will have been arrived at with large letters included in SD in the calculations. eBay needs large letters to be subject to SD in order to keep the average weight down.
They are making enormous profits through SD, so they have headroom to do something. It may make more sense for them to cut prices across the board to bring them slightly under RM prices. A cut of, say 30p, across the board, would still leave them with huge profits, and would give them a little much-needed good publicity.
