My client wants me to do designs of t-shirts and put it in a mock up without any contract. Initially, she hired me on a fixed price per design and after the milestone was finished, she asked me to do new designs and put it in a mock up shirt without a contract. Since she was nice, I did it for several designs. Then, she again asked me to do another set of themes to work with and what she wants now is that she will only send a milestone and pay for it if her customers on her website order it. I told her that that's not how it works but she said that she will not pay for something that will not sell. In fact the concepts came from her. I am just afraid that she will give me bad reviews because I don't want to agree on her terms. What do I need to do for now?
Before you do anything, you should understand that your client is in violation of Upwork ToS.
Upwork does not allow clients to ask freelancers to work for free.
You are not required to tell her this. But YOU should know this.
If you do not have an open contract with this person, then DO NOT start a new one with her, under any circumstances. What she is proposing is proposterous. If you have a current contract with her, then your goal should be to close the contract as amicably as possible.
Because you've already compromised, it's hard to backtrack and refuse these latest requests without friction. It may earn you poor feedback from this client. Yet, it's nothing you can't recover from if you handle things as professionally as possible with this client and others.
To reduce the chances of poor feedback, you could explain that while you understand her concerns about paying for something that possibly won't sell, you can't continue to do hours of work without being paid. Thank her for the opportunity to work with her, suggest that maybe someone else is better suited to help her, and politely end the contract.
Although you are definitely not under Upwork's payment protection, you may also report this client to them in case they'd like to reach out to her regarding her deliberate attempts to get free work.