I found that the cost for laundry is different on every ship, nevertheless soap and bleach from vending machine is $1.00 each.
Between buying soap, bleach wash and dry, you will spend around $5 and $6 per load.
Early morning and any time after 7 PM, excluding the last day of the cruise.
Concerning using the irons, avoid using from 5 PM and 7 PM, everybody has the same idea.
I have purchased laundry supplies in Miami, Fort Lauderdale, Cape Canaveral, Seattle, San Pedro and a few other ports.
I don't advise traveling via air lines with laundry supplies inside your luggage, very bad idea.
Suggestion, purchase your laundry supplies from a dollar store after arriving at your departure Port, if you State side.
1) Powder soap, 2) Powder Bleach, 3) Softener liquid or Sheets, 4) Spotting and stain removers for your entire cruise under $6 for everything.
Granite, not the same as TIDE, but the price is right.
Any left over supplies is a nice bonus for your cabin stuart.