The person that recommends you need to know your work whether you worked with them directly or not. The country should not be a barrier.
You did not mention the nature of the work done by the large US corporation and how long you worked with them. You’ve to present the evidence very carefully to show you worked with the organisation directly, as self-employed, and not an employee of an outsourcing company, nor doing the projects for outsourcing/consulting companies.
Your presentation of the project will also matter.
You can check this thread:
Regarding the payment evidence, if you’ve other strong evidence you can focus more on those. You need to determine who you’re working for, should that be Upwork or the organisation. Will you be able to evidence that you were paid significantly higher than others delivering the same scope of project? Tech nation says salary information, would this classed as salary based on your expertise or one-off project fee?