SQL Sum(Distinct())

Been a while since I've asked an SQL question, so I'm due. :)

I will soon, I think, have a need within an RPG program to get a couple of
pieces of summary information from an order header/detail pair of files.
One piece from the header file and one piece from the detail file. I can
get the 2 pieces separately like this:

Exec SQL
Select SUM(ORDAM)
Into :ORDAM
From ORHDR
Where CUSNR = :CUSNR and
ROUTE = :ROUTE and
STOPX = :STOPX and
SHPDTISO = :SHPDTISO;

Exec SQL
Select SUM(d.ITNSA)
Into :ITNSA,
From ORDTL d Join ORHDR h on d.ONRCU = h.ONRCU
Where h.CUSNR = :CUSNR and
h.ROUTE = :ROUTE and
h.STOPX = :STOPX and
h.SHPDTISO = :SHPDTISO;


I wondered if I could get the 2 pieces with a single statement, so I tried
this:

Exec SQL
Select SUM(d.ITNSA),
SUM(h.ORDAM)
Into :ITNSA,
:ORDAM
From ORDTL d Join ORHDR h on d.ONRCU = h.ONRCU
Where h.CUSNR = :CUSNR and
h.ROUTE = :ROUTE and
h.STOPX = :STOPX and
h.SHPDTISO = :SHPDTISO;

It executed, but the RPG field ORDAM was waAAAyyy off because field h.ORDAM
from the header file was summed for each record in the detail file. I
understand why that is and it makes sense. I did some googling and came up
with this, using Distinct:

Exec SQL
Select SUM(d.ITNSA),
SUM(Distinct(h.ORDAM))
Into :ITNSA,
:ORDAM
From ORDTL d Join ORHDR h on d.ONRCU = h.ONRCU
Where h.CUSNR = :CUSNR and
h.ROUTE = :ROUTE and
h.STOPX = :STOPX and
h.SHPDTISO = :SHPDTISO;

That seemed to me like it would have a problem, because 2 selected order
header records COULD have the same h.ORDAM value. Won't happen often, but
it can and does happen sometimes. If I understand Distinct right, it would
only include one of them in the Sum function. So I did some testing and
found that to be true.

So is there a way to do what I want in one statement? The flip side of that
is, if the statement is very complex, I would rather do it in 2 statements
because I believe in KISS.

Thanks.