Buyer received wrong item - FBA

Hello Everyone,

Today i received a buyers message stating that the wrong item has been delivered (FBA).

The item he received was the wrong flavour.

Im not sure how this has happened and ive checked the following:

1. I dont stock the other flavour and i prep myself and check the items against the listing before shipping. Therefore its unlikely its labeled wrong otherwise he would of received a completely different item and not a different variation.
2. The buyer did actually buy the product.
3. Listing is correct and hasn’t changed.
4. Amazon labels used.

Has anyone had something similiar happen and how do i go about fixing?

Thank you.