This might work if "price" is a String object. If "price" is some other type of variable it will either do unexpected things or fail to compile.
For example, if 'price' is an integer containing 5: "This is your bill:" + price
results in: "is your bill:"
(The 5 is used as an index into the character array.)
A better way to write it might be:
mySerial.print("This is your bill:");
mySerial.print(price);
This will work for any type of 'price' a long as it is printable.