Be very careful namaste how you do that, it is, as Chiyo has stated illegal in many places and is very much against Amex terms and conditions etc.
If anyone reports you to Amex you could be in a whole lot of trouble.
ADD IN :
A few days ago I wanted to buy around 5 DVD's, ok so it is not overly expensive but I did not have the cash on me and so wanted to pay by Amex, the shop at Panthip Plaza which sells original DVD's I found does not accept Amex.
I therefore had a choice, either inconvenience myself by going to an ATM and pulling more cash or just leaving it.
I left them, I did not need them enough, and I prefer the merchant to appreciate the money I am spending no matter where it comes from. Make it difficult and I walk.