total price doesn't display the value - flutter

Question

List cart =[];
double totalPrice = 0.0;
//here is i retrieve cart item from database to calculate
Future getData() async {
final CollectionReference reference = FirebaseFirestore.instance.collection('cart');
reference.doc(FirebaseAuth.instance.currentUser!.email).collection('items').get().then((ds) {
ds.docs.forEach((data) {
/*var temp = {
'image': data['image'],
'name': data['name'],
'category': data['category'],
'price': data['price'],
};*/
setState(() {
cart.add(data['price']);
double e = double.parse(data['price']);
totalPrice = totalPrice + e;
});
print(totalPrice.toStringAsFixed(2));
});
});
}
double getTotalPrice() { // calculate of my total price
double totPrice = 0.00;
for(int i=0; i<cart.length; i++){
String price = cart[i];
double p = double.parse(price);
totPrice = totPrice + p;
}
return totPrice;
}
#override
void initState() {
// TODO: implement initState
super.initState();
getData();
getTotalPrice();
}
Future<void> initPayment({required String email, required double amount, required BuildContext context}) async {
double newAmount = amount * 100;
try {
// create a payment intent on the server
final response = await http.post(
Uri.parse('https://us-central1-market-3387e.cloudfunctions.net/stripePaymentIntentRequest'),
body: {
'email': email,
'amount': newAmount.toString(),
'payment_method_types[]': "card",
});
double roundDouble(double value, int places){
num mod = pow(10.0, places);
return ((value * mod).round().toDouble() / mod);
}
#override
Widget build(BuildContext context) {
bottomNavigationBar: Container(
child: ElevatedButton(
child: Text('Payment', style: GoogleFonts.hammersmithOne(
// i can't display total price value here at amount:
onPressed: () async {
double h = roundDouble(totalPrice, 2);
await initPayment(amount: h, context: context, email: 'test#gmail.com');
},
),
),
I do not know why but i already calculate the total price at getTotalPrice() function, but when i declare var h at the amount, it doesn't display the value cause i think i already did true. can someone show me how it supposed to be the calculation because i already declare in the stripe function.
var h is where i declare my calculation of total price but i doesn't display the value it doesn't working. When i route from shopping cart to payment page , the total price doesn't retrieve and display the value. I already retrieve data from my database from "cart" collection