Just use the round function like this: return round( )*2,1)/2;
For example, returning a discount value. Instead of:
...
return WA_eCart_FormatNumber($totalDiscount, $this->ForceDecimalsC, $this->DecimalPlacesC);
...
use:
...
return round(WA_eCart_FormatNumber($totalDiscount, $this->ForceDecimalsC, $this->DecimalPlacesC)*2,1)/2;
...