I have been looking through all the development ideas and this one stands out. I realize the bulk of development right now is going to making the coins functionality better and cleaning up code, ring CT etc..
You are exactly right the first coin to do this correctly is going to benefit exponentially. I'm no expert in the banking and payment processing world but I have had my own business and had to use credit card processors before. I hate them like many business owners do. They hit you two fold.
First is that each transaction is hit with a gateway fee. Usually .08¢ - .45¢ per transaction. The buying customer never sees this fee and often never knows. Have you ever seen a store that says they charge a fee to process a credit card or we add 3% to all credit card translations, and the most famous is $5.00 minimum for credit card payment. This is why a merchant losses a little money each time they accept a CC transaction but the businesses that put those signs out are trying to recover that cost at the buyers expense. In some ways its tacky to announce this to customers but I can understand why merchants do it.
The Second is the fees that come out of the total sale amount that goes to visa, mastercard, AMEX, Discover and on and on. For example if a item is $100, visa or bank representing visa might get 1.5% to 3% of the total amount. AMEX is even higher on the fees. This is how the CC companies give out rewards so easy to their users. The loser is the business. Each time a payment is charged you are extorted a little by these banks. Its an organized mafia in a way.
So the big problem is how do you beat the gateway fee and how do you get past the bank fees. I'm sure there is a way, but somewhere you have to pay someone for access to the network at least with some forms of fiat transactions. I love the idea of free because I don't believe a business should be punished for doing the one thing they are in business for and that is the selling of an item or service. You have worked hard to bring goods or services to the market, and you have paid the cost of doing business already so why be punished at the very end when you make a sale by being extorted. I understand that business has operating cost but this should not be one of them.
I really like your mock up and design work. Simple and easy to understand and looks intuitively easy to make a payment. The embedded idea of monero is also a winner, and I think it could be a selling point for some from the beginning. However those not interested in the monero aspect in the beginning would still want to use it because its free, and in the future the trojan horse embedding will win over some merchants.
Ultimately free is best for all but to get this rolling is a major undertaking but hey I like thinking big. There might have to be a small fee for transaction to have enough money to pay for software development and for fixing bugs, this may require a network that needs to be employed. Just the minimum needed to pay people a living wage for the work they do. So on fiat currency transaction maybe a ½¢ to .01¢ fee. Not sure how else to ease some of the start up cost this kind of project would or could cost. Maybe a one time fee for joining then all transactions are free.
I wish i was a genius code writer and developer. In this lifetime its not my gift. I do support the idea here in full though. If someone in the know could take this on I'd love to be on board and help anyway i could, hell I'd even be a field agent to turn merchants onto the software just because I think disclosure, transparency, open source and decentralization is the way of the future. Perhaps this POS would have a governing board (un paid) to make sure the charter is followed and costs are kept as low as possible. I do want free overall but right now realistically I'm not sure how it could be pulled off. Like you said which ever coin gets this right and markets it to its fullest capability is going to see a bright future for their coin especially if its monero.
My last thoughts on this for the moment is perhaps there could be an exploratory group of people that could look at this idea from an experts point of view and bring real ideas to the table, but at lest one of the core developers should be on board.