Returning products to Amazon

Amazon told me that they charge a $20 US fee for a return and there is a process to return through DHL.
They also give the option of me finding my own way to return and I just pay whatever that is and they refund it once they get the items back.
I am in no big hurry to get the refund so I think I can get a cheaper option at a post office.

Suggestions? I might just use ground mail…it is for 4 paperback books and very light.