我的代码有问题，我一直在努力解决这个问题。问题是我试图验证我的代码，使它不计算负数。
税收计算器程序的目标是使用C编程来计算每个库德勒美食商店（德尔马、Encinitas和拉霍亚）的销售税
标准输入/输出处理

#include <stdio.h>
#include <ctype.h>
#include <stdlib.h>

//Starting point    
int main ()
{   
//Variables defined
//st = sales tax, tax rated = tr, pa = total purchase amount, sa = purchase amoount
    
    float fstDelmar;
    float ftrDelmar;
    float fpaDelmar;
    float fstEncinitas;
    float ftrEncinitas;
    float fpaEncinitas;
    float fstLajolla;
    float ftrLajolla;
    float fpaLajolla;
    float fsaPurchaseAmount;
    int fStoreselect;
    
//Variable initializations for tax rates and purchase amount 
    ftrDelmar = .0725;
    ftrEncinitas = .075;
    ftrLajolla = .0775;
    fsaPurchaseAmount = 0;

    
//Header & Introduction
    printf("\n***********************************************************************");
    printf("\n*                         Kudler Fine Foods                           *");
    printf("\n*                       Sales Tax Calculator                          *");
    printf("\n*                           Version 3.0                               *");
    printf("\n***********************************************************************\n");
    
    
//Ask user to select store.

    printf ("\n                 STORE LOCATION \n");
    printf ("\n                 1) Del Mar \n");
    printf ("\n                 2) Encinitas \n");
    printf ("\n                 3) La Jolla \n");
    
    printf ("\n Please select the store location (1-3):  \n");
    scanf ("%d", &fStoreselect);    
    
//Validate store selection.
    while (fStoreselect < 1 || fStoreselect > 3) {
        fflush (fStoreselect);
        printf ("INVALID SELECTION! Please select a valid location (1-3): \n"); 
        scanf ("%d", &fStoreselect);
        }
//Ask user to enter in total purchase amount.
  
    printf ("\n What was your purchase amount? "); 
    scanf("$%f", &fsaPurchaseAmount); //user enters variable amount
    
    
//Validation to ensure that user's enter in purchase amounts using correct format.

    while (fsaPurchaseAmount <= 0.0) 
        {
        fflush(fsaPurchaseAmount);                                          
        printf("\n INVALID SELECTION! Please enter a valid purchase amount greater than zero.\n");
        printf("\n The purchase amount is: $ ");
        scanf("%f", &fsaPurchaseAmount);    
        }
    
//Calculation of sales tax in dollars for each of the store locations
    fstDelmar = fsaPurchaseAmount * ftrDelmar;
    fstEncinitas = fsaPurchaseAmount * ftrEncinitas;
    fstLajolla = fsaPurchaseAmount * ftrLajolla; 
    
//Calculation of total sales amount for each of the locations
    
    fpaDelmar = fsaPurchaseAmount + fstDelmar;
    fpaEncinitas = fsaPurchaseAmount + fstEncinitas;
    fpaLajolla = fsaPurchaseAmount + fstLajolla;
    
//Displaying sales amount for purchase for each of the different locations

    switch (fStoreselect) {
    
        case 1: 
    //for Delmar location
        printf("\n       Purchase Amount      Sales Tax     Total Sales Amount ");
        printf("\n       _______________      _________     _________________ ");
        printf("\n            $%.2f              $%.2f            $%.2f",fsaPurchaseAmount, fstDelmar, fpaDelmar);
        break;
    
        case 2:
    //for Encinitas location
        printf("\n       Purchase Amount      Sales Tax     Total Sales Amount ");
        printf("\n       _______________      _________     _________________ ");
            printf("\n            $%.2f              $%.2f            $%.2f",fsaPurchaseAmount, fstEncinitas, fpaEncinitas);
            break;
        
            case 3:
        //for La Jolla location
            printf("\n       Purchase Amount      Sales Tax     Total Sales Amount ");
        printf("\n       _______________      _________     _________________ ");
            printf("\n            $%.2f              $%.2f            $%.2f",fsaPurchaseAmount, fstLajolla, fpaLajolla);   
        break; 
        }
    
    printf("\n Hit the ENTER key to exit program\n");
//Pause the screen and wait for user to hit the ENTER key
    getchar();
    
//EOF      
}