C.P.
Fels Naptha Laundry Bar. About $1 at Walmart.
Get the dress all wet, scrub it with the bar until there is lots of soap in it, and let it sit with the soap on it.
Don't bother rinsing it. Fels Naptha doesn't get sudsy.
Then wash like normal.
It's also a miracle worker for stains. And much cheaper than Shout.