Online purchase - Not correct information

I purchase something from online, got the product and then when i wanted to return this to the store, they told me that the dispatch note and tax invoice should have a green sticker on. I checked and no where is there any green sticker, so I have to send a mail to their online store because they think that I am lying.

Now I have to back and forth to sort this out with the store.

They need to make sure that all products purchased online, should have a green sticker on.

No where on the return policy do they speak of green stickers

Regards Natasha