Hey, I'm Martin from Argentina. Nice to meet you!
I'm a experienced web developer, more than 8 years programming, I've doing a lot: php, javascript, java, android, wordpress. Now I'm working with Python and Django. Always trying to use the better tool for the job.
I just completed a project that integrates a wordpress with paypal and was the exact flow you are looking for but the user was redirected to paypal and then he comes back to the wordpress again. Was not via ajax.
I've got a lot of experience. I use to work doing/maintaining wordpress web sites for non profit organizations. I always tried to make responsive sites.
Just ping me!