there is no way . . . for a seller to "add the fee" automatically to the purchase.
This is not true.
True, he can add the fee to the total cost, but the person receiving the BTC cannot "force" the fee to be paid via the technology.
This depends on how the eventual implementation in the POS is handled.
Thus, the sender could simply not pay it.
This depends also on how the eventual implementation in the POS is handled.
Remember, the transaction gets returned to the wallet if confirmation doesn't take place in 24 hours.
This is not true.
I don't really see Bitcoin transactions as "instant" and impossible to "charge back" as described.
0 confirmation transactions are "instant".
Transactions with confirmations are "impossible to charge back".
The merchant will have to decide what their fraud risk is and how much risk they will accept. Solutions beyond the protocol are possible to increase usability (just like there are additional layers beyond TCP/IP for your internet usage).
Confirmation is necessary, and sometimes it won't happen. You don't have your BTC until the transaction is confirmed. You can forget most businesses taking BTC under these circumstances.
And yet they accept credit cards and PayPal under these circumstances. I think you have a very flawed understanding of what businesses are willing to accept in the interest of increasing profits.
Is there a way for the merchant to "force" the fee to be paid?
Yes.
As for the psychology of "paying the fee," it doesn't matter that we already pay fees to use credit and debit cards. They are currently hidden. It is all in the mind, I admit, but Americans wholeheartedly hate the idea of adding a few cents. Debit/Credit "feels free" and that is what matters.
And bitcoin transactions can be handled that way as well.