make a list of 10-20 online stores that sell what you sell (gold), and research them to identify what payment processor they use.if
you want absolutely zero refunds...might be hard. think there are laws in place to not allow that. there are systems in place to debate an order but so long as you prove you sent it /w tracking, you will win.
reach out to people doing what youre doing and youll find the answers.
simplest and surest way is ditch cash and only accept crypto like bitcoin. lots of gold merchants are starting to adopt btc payments now. payments cant be refunded or debated for any reason.