That's what a Debit card is for.(In Canada, the card you use for ATM transactionscan be used for store purchases, I understand in the USA you must get a card specifically for such transactions.)

Then there is Mondex, which uses an electronic smartcard to store monetary values on the card. With that, you can have person to person transfers, in addition to bank-person and retailer-person.

Debit cards are not hard to use for small amounts and some banks charge for their use. I do not know what a Mondex card is but it sounds like a cash card. The advantage if using a metro card is that there is already cashiers and machines that except cash charge and debit cards all over the city

Thanks for the comment

Two problems I have with cards. One is limited use. In the metrocard case, you put money in it but (I think) can not easily get it back out. The same with gift cards. Someone buys you one that's only good at shoes-a-rama and the closest store is 100 miles away and the card expires and the store steals your gift. Phone cards also expire. The other problem is privacy. Cards when used are stored in a database of who you are, what you bought, and where you bought it. None of their business!

Also, almost all stripe cards use the same format. Metrocard is an exception... it can not be read by bank machines.

A Mondex card is a crad thah holde the cash value in a chip embedded in it, accessable by contacts on it, much like a phone card.

Cards are totally redundant. Cell/mobile phones are the way to go. Just flash your phone past the reader/ATM/cash point, etc. Use your PIN. Monies are immediately withdrawn from your bank a/c and are credited to the stores involved in the transactions. If you lose your phone, it's no worse than losing your wallet, card, or cash.