How to design shopping cart Java application which satisfy modular, extensible and maintainable

I am new in Application Design. I have use-case as below

As per the above story I have implemented code as below without any Modularity, Extensible and Maintainable. Could someone share the thoughts how to design below bad code to production grade application ?

Based on this story context, there could be other stories which could build-upon in later. Example: Introduce new customer types like Gold, Diamond, Platinum, etc and their discounts slabs.

public class ShoppingCartDiscount {

    public static void main(String args[]) {
        System.out.println("Price for the premium customer: " + calculatePrice("Premium", 20000));
        System.out.println("Price for the regular customer: " + calculatePrice("Regular", 8000));
    }

    static float calculatePrice(String customerType, float purchaseAmount) {

        float total = 0;
        if (customerType.equalsIgnoreCase("Regular") {

            if (purchaseAmount > 5000 && purchaseAmount <= 10000) {

                float firstSlab = purchaseAmount - 5000;

                firstSlab = firstSlab - (float) (firstSlab * 0.1);

                total = 5000 + firstSlab;
            }

            else if (purchaseAmount > 10000) {

                float secondSlab = purchaseAmount - 10000;

                secondSlab = secondSlab - (float) (secondSlab * 0.2);

                float firstSlab = 10000 - 5000;

                firstSlab = firstSlab - (float) (firstSlab * 0.1);
                System.out.println("firstSlab:" + firstSlab);

                total = 5000 + firstSlab;

                // 4500 + 800 + No slab
                total = total + secondSlab;

            } else if (purchaseAmount <= 5000 && purchaseAmount >= 0) {

                total = purchaseAmount;

            } else {
                return purchaseAmount;
            }

        } else if (customerType.equalsIgnoreCase("premium")) {
            if (purchaseAmount <= 4000) {

                total = purchaseAmount - (float) (purchaseAmount * 0.1);

            }
            if (purchaseAmount > 4000 && purchaseAmount <= 8000) {

                float secondSlab = purchaseAmount - 4000;

                secondSlab = secondSlab - (float) (secondSlab * 0.15);

                float firstSlab = 8000 - 4000;

                total = firstSlab - (float) (firstSlab * 0.1);

                total = total + secondSlab;
            }

            if (purchaseAmount > 8000 && purchaseAmount <= 12000) {

                float thirdSlab = purchaseAmount - 8000;

                thirdSlab = thirdSlab - (float) (thirdSlab * 0.20);

                float secondSlab = 8000 - 4000;

                secondSlab = secondSlab - (float) (secondSlab * 0.15);

                float firstSlab = 8000 - 4000;

                total = firstSlab - (float) (firstSlab * 0.1);

                total = total + secondSlab + thirdSlab;

            }
            if (purchaseAmount > 12000) {

                float fourthSlab = purchaseAmount - 12000;

                fourthSlab = fourthSlab - (float) (fourthSlab * 0.30);

                float thirdSlab = 8000 - 4000;

                thirdSlab = thirdSlab - (float) (thirdSlab * 0.20);

                float secondSlab = 8000 - 4000;

                secondSlab = secondSlab - (float) (secondSlab * 0.15);

                float firstSlab = 8000 - 4000;

                total = firstSlab - (float) (firstSlab * 0.1);

                total = total + secondSlab + thirdSlab + fourthSlab;

            }
        }

        return total;
    }

}