Use billing address in custom ICustomerTaxClassService

Hello,

Based on the example here, I'm trying to implement a custom ICustomerTaxClassService to determine the tax class.

The only thing I'd like to add is a condition based on the billing address country. In my example, the result should be Taxable, if the CustomerTaxRegistrationID is empty OR if the customer is located in a certain country, regardless of the CustomerTaxRegistrationID.

public class CustomCustomerTaxClassService : ICustomerTaxClassService
{
    private IShoppingService ShoppingService { get; }
    private int UkCountryId { get; set; }

    public CustomCustomerTaxClassService(IShoppingService shoppingService)
    {
        ShoppingService = shoppingService;
        UkCountryId = CountryInfoProvider.GetCountryInfoByCode("GBR").CountryID;
    }

    /// <summary>
    /// Returns a CustomerTaxClass object for the specified customer to determine whether they are subject to tax.
    /// The default value is CustomerTaxClass.Taxable.
    /// </summary>
    public CustomerTaxClass GetTaxClass(CustomerInfo customer)
    {
        var billingAddress = ShoppingService.GetBillingAddress();
        if (billingAddress != null && customer != null &&
            (billingAddress.AddressCountryID == UkCountryId || string.IsNullOrWhiteSpace(customer.CustomerTaxRegistrationID)))
        {
            // Apply tax rules if customer billing address is UK or customer doesn't have tax registration ID
            return CustomerTaxClass.Taxable;
        }

        return CustomerTaxClass.Exempt;
    }
}

The problem is, calling ShoppingService.GetBillingAddress(); causes a StackOverflow exception, because that call by itself seems to perform a shopping cart calculation, which will eventually call the GetTaxClass method again.

Is there a better solution to do this or a way to avoid the exception?

Thanks, Andy