Most golf caps that get thrown away are not worn out. They are dirty, and they have been through a wash that ruined the shape.
Those are two different problems. Only one of them means you need a new cap.
The sweat marks are the part people notice. After a summer of eighteen-hole rounds the front panel goes pale and stiff along the top of the brim. That white line isn't fading. It's salt, left behind as sweat dries into the fabric, round after round.

Salt washes out. That's the easy part. The problem is what happens to the cap on the way.
What actually ruins a cap
It isn't the water. It isn't the detergent. It's the spin.
A drum turns at high speed at the end of every cycle. Anything loose inside gets thrown against the wall of it, over and over. Most laundry is soft, so it doesn't matter.
A cap is not soft. The peak is a rigid panel, and it takes every one of those impacts on its edge.
That's what bends it. The brim comes out wavy instead of curved. The crown collapses because the fabric has been folded against itself at speed. The seams that hold the shape go first. Do it once and you might get away with it. Do it twice and you probably won't.
The label says do not wash. The label is right — about washing it loose.
That's where people stop. They read "do not wash", decide caps simply can't be cleaned, and leave it. So the cap sits in the hall getting worse, and eventually gets replaced by a new one that goes the same way.
What would actually work
If you're putting a cap through a machine, three things have to be true at once. Most methods manage one or two.
What I tell people to use
NordCap is a rigid frame shaped like a cap. The cap goes inside, the frame closes around it, and the whole thing goes in with the rest of your wash.
The difference is at the front. A ribbed grid runs along the peak and holds the brim in its curve for the whole cycle — including the spin, which is when the damage happens. The frame takes the impacts so the cap doesn't. And because it's built from open ribs rather than solid panels, water and detergent still reach the fabric everywhere.
It protects and it cleans. That's the part a bag can't do.

What comes out
Before
Salt across the peak, stiff fabric, a smell you stopped noticing. Wearable, but only just.
After
Clean fabric, no salt line, and the same curve in the brim it had going in.
Let it air dry in the frame and the shape holds through drying too — which is where plenty of caps get ruined even after a careful wash.
Then you do it again next month. That's the actual point. Not one rescued cap, but a cap you keep wearing for years instead of replacing every summer.
A decent golf cap costs about forty. The frame costs less and lasts every cap you'll ever own.
365 days to change your mind
Try it on your own caps, in your own machine. If washing them doesn't get easier and the shape doesn't hold better, send it back within the year for a full refund.
Questions people ask in the store
Will it fit my cap?
It fits most standard caps — baseball caps, snapbacks and truckers. The frame is roughly 34 × 22 cm outside, with an inner space of about 30.7 × 21 × 11 cm.
What if I've got a front-loading machine?
Gentle cycle, low temperature, and the frame goes in closed. If you'd rather not use the washing machine at all, it works in a dishwasher too — top rack, no drying cycle.
Does it work with normal detergent?
Yes. Use whatever you already wash with. Nothing special to buy.