Lots and lots of people are psychologically trained to expect that they pay the sticker price for things. Yes in recent times some places have started to show additional fees for using credit cards, but only as a way to try and push people onto cheaper forms of payment, not because customers love surcharges. It's for that reason that many merchants don't do this, even when they could - I pay for things with Maestro Debit all the time and I don't think I've ever seen a surcharge. That system obviously has costs, but they're included.

This is just a basic cultural thing - when I buy something from a shop, the social expectation is that the seller should be grateful for receiving my money. "The customer is always right". When I send money to a friend, the social expectation is different. If my friend said, hey Mike, could you send me that 10 bucks you owe me from last weekend and what he receives is less than 10 bucks, he would probably feel annoyed - if I owe him 10 bucks then I owe him 10 bucks and it's my job the cover the fees. That's why PayPal makes sender pay fees in that case.

Maybe we need new terminology for this. Interior fees for included in the price/receiver pays and exterior fees for excluded from the price/sender pays?

Have you thought through the UI for that in detail? How exactly are you going to explain the fee structure? Let the user pick the number of blocks they need to wait for? What's a block? Why should I care? Why shouldn't I just set the slider all the way to the other end and pay no fees at all? Is the merchant going to refuse to take my payment? Gavin just said that's not possible with Bitcoin-Qt. I'm thinking for bitcoinj I might go in a slightly different direction and not broadcast payments submitted via the payment protocol (and definitely not have one wire tx pay multiple payment requests simultaneously, at least not for consumer wallets).