I've seen a lot of topics for PayPal integration for sites, Java, C# and even Objective-C, but I can't seem to find anything on C/C++, specifically. So where can I look to get an idea of what might be necessary to go about adding PayPal into a C++ project that is intended to operate on Windows, Mac and Linux, at least?
The objective is to create an in-project store, along with an in-project donation jar of sorts. Is there a viable cross-platform solution, or does it need to be changed based on platform? Either way, is C/C++ still a possible language to write these per-platform systems?
Well, there is no official NVP library for C++ and I doubt using a third-party one (if it exists) is a good idea.
So, you should probably go with their SOAP API. Finding SOAP libs for C++ is not a problem.
Another option is simply using a webpage and opening it (that is, in the user's browser). Or you could integrate some HTML viewer.