createCustomer method
Store a customer's details in a customer object to reuse in future payments.
Link a payment instrument using the Update customer details endpoint,
so the customer id
returned can be passed as a source when making a payment.
Implementation
@override
Future<String?> createCustomer(Customer customer) async {
//
final Map<String, dynamic> responseMap = await apiBase.call(RESTOption.post,
resource: _customers, headers: headers, body: customer.toJson());
return Customer.fromMap(responseMap).id;
}