You must have missed where I said it was bringing your clothes into Kona that could be the Customs problem; not back to the ship. It's US Customs who might get involved.
Are you aware that Kona is a tender port, and even on days when you can get ashore, it can be very difficult to board the tender? Adding in a suitcase to deal with just adds to the danger (although the crew might be helpful.)
As Jayhawk said, you can get an unlimited laundry package for $7/day times the number of days in the cruise. That means it costs you the same to send laundry every day, with very little in it, as it does to wait until a bag is full. Come in from your day, change for dinner, and send what you want right then. It will likely be back the next afternoon (or sooner).
But in the US, the bag at a time price is $20, not the $30 posted (unless it went up since cruising started again, but I haven't read that it did.)