Former SMP student here.
Yes, there are ones that guarantee admittance provided that you maintain a certain GPA and sometimes pass a comprehensive final. There are others that guarantee an interview, and others that are there to beef up your application.
I think you´ll hear this a lot, but make sure you do your research. Not all SMPs are the same. With some it is common for most to get As, with mine, we were graded against each other and it was very difficult to get As. 7 or 8 of my classmates dropped out during the first semester, another 5 or 6 during the second.
In my opinion, SMPs are expensive and should be a last resort. I don´t know much about post baccs, maybe someone else can comment on that.
Either way, feel free to PM me if you have any other questions and good luck to you!