At the risk of setting a bad example for less experienced freelancers, I frequently use the "alarming" business model another freelancer described above.
Many of my clients really hate Upwork, and employing a hybrid system wherein the only reason they ever have to log in is to make a payment when the job is done allows me to comply with Upwork's requirement that payment passes through the platform while also minimizing the client's need to spend time on the platform. It also creates a greater likelihood that the client will be willing to continue processing through Upwork after the two-year restriction expires.
I definitely don't recommend this as a regular practice, and definitely not with a client who is still an unknown. But, every client/freelancer relationship is different and operating a business requires judgment calls. So far, I've not only been paid every time, but clients tend to log in to make the "bonus" payment more quickly than they did to approve milestones. There's a risk, sure, but everything is a trade-off.
It looks like you were already paid for the first part of the project. As some of the community members have suggested, please make sure that a milestone is created, and the client's payment method is verified, and funded before you start working on any project for any client.
Also, I would suggest that you only submit your completed work via the milestone-submission/contract page.
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