First, I will put the code here then explain all the changes I made (your original attempt was very close, just a few tweaks):

#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. As @Devang Sanghani mentioned, you should call your main() outside of your function so that it will run

2. For your calc_subtotal() function, use a while loop to keep taking in user input until given a valid number. Make sure you move your return subtotal outside of this loop so that it will only return it once this number is valid.

3. In your calc_shipping() function, make sure you take in subtotal as a parameter since you are using it in your function. Make sure you also return shipping.

4. Similarly, in your calc_total() function, take in both subtotal and shipping as parameters since they are used in your function.

5. Given these changes, update your main() function accordingly.

I hope these changes made sense! Please let me know if you need any further help or clarification :)

So there seem to be 2 errors:

1. You forgot to call the main function
2. You indented the return statement in calc_subtotal() so it returns the value only if the subtotal <= 0 The correct code might be:

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

Adding to the above answers, also consider the case where the price can be a few cents, so this line should change:

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