It is incredibly hard to win as a merchant in any kind of dispute. The burden of proof is on the merchant not the customer.
The merchant pays a fee regardless of the outcome, and if you end up with more than 1% chargebacks your account is cancelled. The net effect is that any merchant who is doing this on a regular basis won't be accepting credit cards for long.
Additionally, the system is so far in the customer's favor that it's causing a fairly large fraud problem for merchants. It's bad enough that my small business won't accept credit cards for very large purchases unless the customer has a history with us.
You need documented signed proof, in general and without that the charge is not only backed out but you get hit with a $25 or so charge back fee. And yes to many chargebacks say goodbye to the account.
We spot fraudulent charges and proactively issue a void before the consumer finds out, lest we get hit with the charge back fee as well.
A company I used to work for had, some years back, made its way onto MATCH (merchant blacklist) for one of the four major credit card companies. Why? It accepted a legitimate (!) payment from a customer who was at the time under 'investigation' by the credit card company for fraud. (Obviously, this was not known to my company at the time that they accepted the payment).
Apparently, we even showed the credit card company that we had provided the appropriate services in exchange for the payment, so there could be no allegation of us helping them 'embezzle' the money. No luck - we were placed on the blacklist for seven years (give or take).
The punchline: that same credit card company had no problems issuing us a credit card in the company's name - not just once, but three times since we were placed on the blacklist.