Need Someone Extremely Experienced with Connecting PayPal (and Authorize.net) with Membership Site

Hi there, I am working with software that is for a membership/subscription type website. It's supposed to be set up with both PayPal and [login to view URL], but I'm having a problem. While testing, when I make a payment through these payment processing companies it's not communicating back to my site that they've paid. They should be getting upgraded, and all this information should be relayed back to their account in my database, and they should see it on their User Profile. etc etc I've been told by the developers that their product works only with DPM (Direct Post Method), so I'm not sure what that means. I've asked the original company to fix it, but they've been "working on it" for almost 2 months now and I can't lose any more time. Please only respond if you have had experience with something like this before. The problem is not in setting up the PayPal functionality on the front end, but on the back end - where it communicates with my site. Thank you, Emily