With around 3 million sellers worldwide you can try to understand the logistical nightmare Amazon has become. The volumes involved over the globe are huge. It is very rare for any small time retailer to get a monthly invoice that is correct 100% even Amazon admits that. If you do not pay attention you as a seller could be loosing a lot of money.
Amazon does have checks and balances put in places but still some one has to keep an eye, so they have a refunds department just for that purpose called Amazon refund services. They allow you to file a complaint with them for up to 18 months after the transaction is made.
There are a lot of reasons why small sellers need reimbursements the more important thing is they are small and cannot afford to loose any money in this day and age. To remain profitable they have to offer very competitive prices, to do that they need to maximize their profits as the margins are really small.
There is whole set of rules to follow to file for a refund at Amazon for these retailers there are people in special department there who take care of such requests and help you with the proceedings. They have people working at amazon specialist teams to handle such situations in a timely manner. Alongside of providing business consultancy to their small sellers Amazon also makes sure their retailers are take care of in case of any discrepancy and they want to resolve any issue as fast as possible. So they have this specialist team standing by ready to take care of all your needs. Amazon has a reputation to keep it did not get where it is without nothing and they make sure everybody using their services be it the buyer or the seller are satisfied.