Buyer did not choose correct shipping option

Good Morning,

I am currently selling multiple clothing items on Trademe and offer a $1 combined postage fee if a buyer is wanting to buy multiple items and combined the shipping. 

Last night, a buyer purchased 1 item and chose that shipping option. I emailed AND messaged them on Trademe letting them know that the shipping option they chose was incorrect and I asked if they were wanting to purchase something else and pay the full shipping on the other item. If not, I would send them a full refund and they can choose to repurchase the item with the correct shipping amount if they would like to. I was ignored by the buyer and was going to give them more time to respond before I gave them a full refund.

But this morning I woke up to positive feedback from the buyer (which means they have been on Trademe and may have seen my message), and also an email from them with the title "Please please sorry please send me my feedback thanks". Nothing in the body of the email, only the title. I felt a bit strange about the whole thing since I knew then that they would have seen my email and ignored it. 

So I sent them a full refund and requested my success fee back from Trademe and I also added them to my blacklist. 

However, I am worried now that they will change their feedback to negative since I didn't play ball and just send them the item even though they didn't pay the correct shipping amount. Since I blacklisted them, are they still able to change the feedback in retaliation? 

It's definitely something I have never come across on Trademe before, it seems like they were wanting to work up their positive feedback????