Thread: Tiered Coupon

I am looking to add a tiered coupon to our website (i.e. $5 off $25+, $10 off $50+...). I have added a field to the coupon table that is "CouponTiered". This is a 1 or 0 field. In coupons.cs I added the following:
public class CouponObject

Code:

internal bool m_coupontiered = false;

public List<CouponObject> InitCoupon

Code:

co.m_coupontiered = DB.RSFieldBool(rs, "CouponTiered");

Code:

        public bool CouponTiered
        {
            get { return m_coupontiered; }
        }

public static CouponObject GetCoupon()

Code:

co.m_coupontiered = DB.RSFieldBool(rscoup, "CouponTiered");

public static String CheckIfCouponIsValidForOrder()

Code:

if (status == AppLogic.ro_OK)
                {
                    if (co.m_coupontiered && st < 25)
                    {
                        status = String.Format(AppLogic.GetString("shoppingcart.cs.73", ThisCustomer.SkinID, ThisCustomer.LocaleSetting), ThisCustomer.CurrencyString(25));
                    }
                    // If we get here, we need to change the discount amount to match the tiered pricing
                    else
                    {
                        if (co.m_coupontiered && st >= 25)
                        {
                            if (st >= 25 && st < 50) // $5 off $25+
                            {
                                co.m_discountamount = 5;
                                co.m_description = "$5 off $25+";
                            }
                            else if (st >= 50 && st < 100) // $10 off $50+
                            {
                                co.m_discountamount = 10;
                                co.m_description = "$10 off $50+";
                            }
                            else if (st >= 100) // $20 off $100+
                            {
                                co.m_discountamount = 20;
                                co.m_description = "$20 off $100+";
                            }
                        }
                    }
                }

With this code, I can type the coupon code in on shoppingcart.aspx and click the update button to the right, and it will show the $X off $X+ but I have to push the update button again for it to reflect in the subtotal. Any ideas why this is or if there is a better way to do this?