Corrects coupon rounding behavior in Pricing.calculations

##### previously
```
subtotal * rate = discount
19.95 * 0.5 = 9.975

subtotal - discount = subtotal after discount ~> rounded
19.95 - 9.975 = 9.975 ~> 9.98
```
*9.98*

##### now
```
subtotal * rate = discount ~> rounded discount
19.95 * 0.5 = 9.975 ~> 9.98

subtotal - rounded discount = subtotal after discount
19.95 - 9.98 = 9.97
```
*9.97*

cc @cbarton