So a lot of designers/artists like you and I had heard that, or even said that. 

When I work on a graphic and submit it the requester, it sucks if they don't use it but they have a right NOT to. It's their story, they totally have the right to decline. You as the artist, should suck it up and move on or retry. 

Now here's the second part: they undo their payment. 

That sucks. The y say the following:

"In real life, you return the product, and you get a refund,"

 OR 

"I don't have to pay for something I didn't use just like I'm not forced buy something in a store."

For a bit, that sort of made sense to me. It was a good argument even though I disliked it. But here's the truth: You aren't in a fucking store/shop. 

Threads/Graphic 'shops' aren't really a 'store' in comparison to real life. They are like restaurants. 

What you are 'buying' in wattpad graphic shops is the artist's SERVICE and not the graphic. Service is the  time an artist/designer put into their time in creating the cover/graphic. Their time is what your are 'paying' for. 

So don't be an ass by undoing the payment. 

Still not convinced? Any professional graphic designer can tell you that you are paying for their service not the artwork. I talked an art director who works for a very well known publisher, and she was saying how she had to turn two artwork from two different artist down because it didn't fit the book. But guess what? The artists still got paid. 

That being said. Don't have payments that can be easily undone like a follow. It takes less and 2 to undo it. There's no point.