Help With SQL If Statement And Adding Fields Together

I have a query that I need a hand on. I am trying to add togther some
fiends based on values of another.

What I would like to add a billing total by saying more or less the
following:

SELECT labor_hours, labor_cost, expidite_fee, flat_rate,
include_repair_cost, include_cal, include_flat_rate, include_parts,
cur_bill,
(labor_hours * labor_cost) AS labor_total,
(ISNULL((SELECT TOP 1 cal_cost FROM calID WHERE orderID=79559 ORDER BY
dateCAL DESC),0)) AS cal_total,
(
ISNULL((SELECT SUM((qty * cost) + premium_charge) AS gptotal FROM
repair_partsID WHERE orderID=79559),0) +
ISNULL((SELECT SUM(qty_needed * cust_cost) AS gnptotal FROM
misc_part_assocID WHERE orderID=79559),0)
) AS parts_total,
(
(labor_hours * labor_cost) + expidite_fee + flat_rate +
ISNULL((SELECT TOP 1 cal_cost FROM calID WHERE orderID=79559 ORDER BY
dateCAL DESC),0) +
ISNULL((SELECT SUM((qty * cost) + premium_charge) AS gptotal FROM
repair_partsID WHERE orderID=79559),0) +
ISNULL((SELECT SUM(qty_needed * cust_cost) AS gnptotal FROM
misc_part_assocID WHERE orderID=79559),0)
) AS actual_total,
(
expidite_fee
IF include_repair_cost = 1
+ (labor_hours * labor_cost)
IF include_flat_rate = 1
+ flat_rate
IF include_cal = 1
+ ISNULL((SELECT TOP 1 cal_cost FROM calID WHERE orderID=79559 ORDER
BY dateCAL DESC),0)
IF include_parts = 1
+ ISNULL((SELECT SUM((qty * cost) + premium_charge) AS gptotal FROM
repair_partsID WHERE orderID=79559),0) +
ISNULL((SELECT SUM(qty_needed * cust_cost) AS gnptotal FROM
misc_part_assocID WHERE orderID=79559),0)
) AS billing_total
FROM view_inventory
WHERE orderID=79559

I know the IF part is whacked, that's where I need the help. Is this
type of thing even possible? Or even efficent? Is it wise to subquery
for totals (not like I have a choice based on the application
requirements)? help.