Thank you all who have replied.

As I said previously, an average sale could be a day's pay for an average (Filipino) worker. Not an insignificant amount. The shop employee handling the transaction would have just a basic (Filipino) high school education. This would mean that he/she can do no more than follow basic instructions - if your lucky.

Neither the sales person or the customer are going to wait for even the first confirmation. The merchant (shop) cannot set or easily check the transaction fee. Let's assume that the customer makes the purchase with zero transaction fee. (I know, Bitcoin Wallet does not allow this.)

So, the customer walks in, buys a coffee and food (take-out) with bitcoin zero transaction fee, then leaves the shop. How does this customer "double spend" his/her bitcoin?

What is "another spend that uses the same inputs"?