Get it in writing whenever you are involved in any business transaction that is not an arms length exchange with a customer
Many people complain it would be too much work. But depending on the size of your business, you must develop a dollar amount at which paperwork should be required. If you’re in the first few months of your operation, your work will not have become so great that you can’t take the time document most things. And, get the signature of any person you are making an agreement with.
When you pay minor operating costs you will usually get a receipt. But, if you felt you could not recover from a loss on a $500 purchase, get the terms in writing. You’d be laughed out of a court of law, should you seek damages but could not prove harm. Settlement does not have to be financial, it just has to be what will pacify you. If a vendor promised substantial after market service he failed to deliver, you would have been harmed. A settlement could be the delivery of the after market service as promised. But if you don’t have it in writing, you have no claim.
Larger, and financially secure businesses should choose different thresholds for when a transaction cannot be consummated, unless it is reduced to writing. Keep in mind many small transactions can become something big. That’s why you have overcharge auditors. But without proof you have no case for reimbursement.
It would be embarrassing if anyone else knew about your misfortune, but it’s rare you can bring a cause of action, without proof.