I think it will have more to do with the margins in the merchant's category than transaction size. I don't foresee full service department stores or restaurants adding a surcharge. However, I think off-price retailers, and those battling direct internet competition (e.g. books, recordings, electronics) will do something, but it may not be heavy-handed. Bear in mind that many large retailers are also partnered with card-issuing banks, and they may cut a deal which encourages use of their own card, or cards from the same issuer.
Overlayed on this will be the elimination of sales tax exemptions for many internet sellers. While logically separate, I think the two changes will occur simultaneously and the retail environment will look very different in a decade.
What about a merchant with many items that have different profit margins?
I can buy medicine at CVS and the profit margin on that is big.
Or, I can buy GCs there and the profit margin will be low.
What happens then, if all surcharges must be equal?