I still would not recommend automatic approval UNLESS you are not capable of handling prompt replies.
A well worded return authorization allows you to accomplish several major things:
Establish it was originally received on time and as described (and state the serial number and specs/description as appropriate)
Establish the reason for return, especially if it is buyer discretion
Establish that you are following Amazon policy and authorizing return
Set the terms and conditions of return - such as packaging and arrival time and the fact that it has to be returned unopened/original condition as appropriate, with all parts and packaging, that it has to be sent with tracking and/or signature
Establish that you will refund promptly on receipt of the return as per Amazon policy
Optionally, establish that Amazon policy provides for a reasonable restocking fee for items returned opened, used, damaged or missing items.