One of my best upvoted stackoverflow answers is about a paypal sandbox bug (or "undocumented feature") where it would disregard the port in the pingback url.
Just generally the sandbox was horrible, it had a weird 'double login' system where it often got confused between whether you were 'inside' the sandbox logged in as a fake user or outside logged in as your paypal account.
And you're right about their woefully confusing documentation, it was definitely hard to understand which payment flow you were trying to use and which was appropriate.
I suspect also that the marketing folks re-branded the various services on a regular basis so you weren't even sure if two different names for something actually referred to the same thing.
Just generally the sandbox was horrible, it had a weird 'double login' system where it often got confused between whether you were 'inside' the sandbox logged in as a fake user or outside logged in as your paypal account.
And you're right about their woefully confusing documentation, it was definitely hard to understand which payment flow you were trying to use and which was appropriate.