Making payment methods dependent on shipping options

using CMS;
using CMS.Ecommerce;
using CMS.Base;

[assembly: RegisterCustomProvider(typeof(CustomPaymentOptionInfoProvider))]
public class CustomPaymentOptionInfoProvider : PaymentOptionInfoProvider
{
    protected override bool IsPaymentOptionApplicableInternal(ShoppingCartInfo cart, PaymentOptionInfo paymentOption)
    {
        // Does not check availability if the shopping cart or payment option is not available.
        if ((cart == null) || (paymentOption == null))
        {
            return true;
        }

        // Allows all methods except PayPal.
        if (!paymentOption.PaymentOptionName.EqualsCSafe("PayPal"))
        {
            return true;
        }

        // Forbids PayPal within USA.
        if ((cart.ShoppingCartBillingAddress != null) &&
            cart.ShoppingCartBillingAddress.GetCountryThreeLetterCode().EqualsCSafe("USA"))
        {
            return false;
        }

        // Forbids PayPal if the USPS shipping method is selected.
        return (cart.ShippingOption == null) || !cart.ShippingOption.ShippingOptionName.EqualsCSafe("USPS");
    }
}