Do you just want a basic cross-OS client, and only for file upload to S3Bubble? That seems fairly straightforward.
I don't see your API on S3Bubble, so perhaps you can tell me more. I'm guessing the upload will happen to S3Bubble's S3 account? And then the client tells S3Bubble the location and the user ID or something so you can file it under that user in your own database. Is that roughly correct?
If you're not too picky about the user interface, I'll be able to produce a basic program that does this in a week. If you're fussy about graphics, fonts, widget positions and so forth, it will take a couple of weeks more.
I'll charge £2500 for a basic program. £7500 if you want to dictate every pixel of the GUI. Is that roughly within your budget?
I've done AWS EC2 development, and written network protocol clietnts and servers, and basic GUI applications that ran on Linux, Windows and Macs.