There are two sides to every story, and the truth usually lies in the middle. Since we have only yours here, here’s what I would do.
Consider the costs and benefits of collections relative to your cost (not price) of whatever good or service you provided. Don’t forget opportunity costs of posting and reading here, for example.
Then offer some sort of discount on the current item or some other way of incenting your customer to pay. It will surely cost less to accept less now than wait for potentially collecting less or nothing during collections. You don’t have to detail exactly what happened or why, but take responsibility and be humble in asking for payment. Give them 30 days max, and then send a formal demand letter if you do that yourself; otherwise the collections agent will do it for you.
Consider the risks of negative reviews or publicity if you pursue collections. And, consider the customer’s side of the story as well. I don’t necessarily subscribe to the theory that the customer is always right, but we know that at least some of the fault is with you.
If the customer is in a different state, your collections cost will be higher and will take longer, so consider that expense.
Put a time limit on your efforts to collect (45 days would by my max) and then turn it over to your collections agent with all that supports your side. Learn from whatever went wrong on your end and move on.
You might consider a payment intermediary or agent going forward. As much as I despise PayPal, it’s a well known one that might intervene in a case like this.