首先，我将把代码放在这里，然后解释我所做的所有更改（您最初的尝试非常接近，只是一些调整）：

#Assignment shipping calculator
#Author: Name

def main():
  print("Shipping Calculator")
  subtotal = calc_subtotal()
  shipping = calc_shipping(subtotal)
  calc_total(subtotal,shipping)

def calc_subtotal():
    subtotal = float(input("Cost of Items Ordered: $"))
    while subtotal <= 0:
        print("You must enter a positive number. Please try again.")
        subtotal = float(input("Cost of Items Ordered: $"))
    return subtotal

def calc_shipping(subtotal):
    if subtotal >= 1 and subtotal <= 29.99:
        shipping = 5.95
        print("Shipping Cost: $5.95")
    if subtotal >= 30 and subtotal <= 49.99:
        shipping = 7.95
        print("Shipping Cost: $7.95")
    if subtotal >= 50 and subtotal <= 74.99:
        shipping = 9.95
        print("Shipping Cost: $9.95")
    if subtotal >= 75:
        shipping = 0
        print("Shipping Cost: Free")
    return shipping

def calc_total(subtotal,shipping):
    total = subtotal + shipping
    print("Total Cost: $" + str(round(total,2)))
    print("Thank you for using our shipping calculator!")

main()

1.正如@ DevangSanghani提到的，应该在函数外部调用main()，以便它能够运行
1.对于calc_subtotal()函数，使用while循环接收用户输入，直到给定一个有效的数字。确保将return subtotal移出此循环，以便它只在此数字有效时返回它。
1.在calc_shipping()函数中，确保将subtotal作为参数，因为您在函数中使用它。确保还返回shipping。
1.类似地，在calc_total()函数中，将subtotal和shipping都作为参数，因为它们在函数中使用。
1.考虑到这些更改，请相应地更新main()函数。
我希望这些改变是有意义的！如果你需要任何进一步的帮助或澄清，请让我知道：）

除了上面的答案，还要考虑价格可能是几美分的情况，所以这行应该改变：

if subtotal >= 0 and subtotal <= 29.99: # changed from 1 to 0

因此，似乎存在2个错误：
1.您忘记调用main函数
1.您缩进了calc_subtotal()中的return语句，以便它仅在subtotal <= 0

def main():
    print("Shipping Calculator")
    subtotal = calc_subtotal()
    shipping = calc_shipping()
    total = calc_total()

def calc_subtotal():
    subtotal = float(input("Cost of Items Ordered: $"))
    if subtotal <= 0:
        print("You must enter a positive number. Please try again.")
        calc_subtotal() # Re asking for the input in case of invalid input
    else:
        return subtotal # returning subtotal in case of valid input

def calc_shipping():
    subtotal = calc_subtotal()
    shipping =  calc_shipping
    if subtotal >= 0 and subtotal <= 29.99:
        shipping = 5.95
        print("Shipping Cost: $5.95")
    if subtotal >= 30 and subtotal <= 49.99:
        shipping = 7.95
        print("Shipping Cost: $7.95")
    if subtotal >= 50 and subtotal <= 74.99:
        shipping = 9.95
        print("Shipping Cost: $9.95")
    if subtotal >= 75:
        shipping = 0
        print("Shipping Cost: Free")

def calc_total():
    total = calc_subtotal() + calc_shipping()
    print("Total Cost: ",total)
    print("Thank you for using our shipping calculator!")
main() # Calling main