README.md

Test cards to simulate a payment:
Payment succeeds                  4242 4242 4242 4242
Payment requires authentication   4000 0025 0000 3155
Payment is declined               4000 0025 0000 3155

4000002500000003 	tok_fr 	pm_card_fr 	France (FR) 	Visa

```
P = N + 0.25 + N*f
  = N*(1+f) + 0.25

N = (P - 0.25) / (1 + f)

10*(1+1.4/100) + 0.25 = + 0.39
100                   = + 1.65
500                   = + 7.25
```