Rude Buyer wants to cancel after I shipped

This buyer was pretty demanding from the start but I ignored the red flags cause they said they really liked the pillow and that they needed some time to contact their bank to make the purchase becuase their card was declining. The item was $35 I offered them $31 and everytime I made the offer it was becuase they asked me to send the offer cause "they were ready" but like 4 times they said they were having issues with their card which I understand so I was patient. But they were kinda demanding telling me " you need to be patient" like girl who are you ??? When they finally made the purchase I get a message from them after it's already shipped telling me what flaws do the pillow have ? I didn't read the description? At this point I was done being nice to them cause what do you mean you were trying to buy the pillow for a week and you never once read the description? That isn't my fault and I can't do anything about it cause it's shipped . They also won't be able to return it if they claim "not as described " because the description states the flaws that the pillow has. It's not a mystery that there's some flaws BECAUSE ITS IN THE DESCRIPTION... I drop off my packages with mailboxes so the post office usually picks it up within a day.. because of that she's accusing me of lying and saying that I haven't shipped yet even tho I did and I don't have the pillow anymore. I wouldn't lie if I hadn't shipped yet I would've cancelled cause she's disrespectful and not responsible enough to read a description. I told her after work I can try to see if the package is still at the mailbox I where I left it ... but should I do anything for her at this point ? Isn't it on her that she didn't read the description. I'm worried she'll rate me negatively or try to lie to mercari to get a refund or something. It doesn't seem like this would be a smooth transaction and honestly I'm nervous .