Ok, I cannot get this. I've looked at it and I don't see why it's out of bounds. I get the error at paypalItems[paypalItems.Length] = new PaymentDetailsItemType

        PaymentDetailsItemType[] paypalItems = new PaymentDetailsItemType[order.OrderItems.Count];

    for (int i = 0; i < order.OrderItems.Count; i++)
    {

        paypalItems[i] = new PaymentDetailsItemType
        {
            Name = order.OrderItems[i].Name,
            Amount = ApiUtility.CreateBasicAmount(order.OrderItems[i].Price),
            Description = order.OrderItems[i].Name,
            Number = order.OrderItems[i].Sku,
        };
    }

    if (giftCardsTotal != 0)
    {
        // add Coupons & Discounts line item
        paypalItems[paypalItems.Length] = new PaymentDetailsItemType
                                                  {
                                                      Name = "Gift Cards",
                                                      Amount = ApiUtility.CreateBasicAmount(giftCardsTotal),
                                                      Description = "Gift Cards"
                                                  };
    }