There are any number of self-help books out there that teach assertiveness. That being the case, it doesn't make sense for anyone to write a book here to teach you assertiveness. Go shop around for a book that seems useful to you.
In a book on assertiveness, you will learn certain techniques. An example is the broken record method. You bring an item back to the store to return it. They don't want to accept the return.
Store: You can't return this item without a receipt.
You: I don't have a receipt because it was a gift, but I want to return it anyway. I will accept either a refund or a store credit.
Store: It's our policy to accept no returns without a receipt.
You: I understand that, but you do sell this very product, it is on shelves now, I am returning it with all the original packaging, and I have done everything right that I could have done. I want to return it anyway. I will accept either a refund or a store credit.
Store: If you don't show us a receipt, how do we know it wasn't stolen, or purchased from a different store?
You: I understand your concerns, but I want to return it anyway. I will accept either a refund or a store receipt.
The words in boldface are the repeating parts. Eventually you wear the clerk down and they accept the return.
I just realized some of you might not understand the metaphor of a broken record. Back in the days of vinyl records, when there was a scratch on the vinyl, the same little bit of music or speech would repeat over and over and over.
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A finger in every pie.