To be honest, yours is quite an unusual complaint - normally, it's another carrier who gets a lot of flak for delivering damaged items.

So, first of all, have a think about how you package items.  Be honest with yourself and ask if any of the fault lies with you.

I always think that, if a parcel is going to stand being thrown into a sack, have mulltiple heavy items piled on top before being chucked into a van, squashed by multiple other sacks full of parcels, and then unloaded onto a hard floor before moving at high speed along a mechanised belt, it should be so secure that I would happily throw it down the stairs before I sent it off.

Then, as advised, make sure if an item does arrive damaged, you make a claim against Royal Mail.

If buyers open a return, don't be so quick to replace or refund.  A lot of the stuff you sell looks as if it could still be used if the container arrived damaged.

Offer a partial refund.

Ask for a picture of the damage.

Royal Mail put seriously damaged packages in a plastic RM bag - ask the buyer if it was in one.

Hopefully (some of) my advice is useful.

Genuinely, good luck with your business and your health.