Now hold on. IF this certificate will be used across all versions of SketchUp, then yes you CAN have your own application folder in the user %AppData% path.
"C:/Users/Joe/AppData/Roaming/YourCompany/ThisPlugin" (Windows)
If it is for any user on a workstation, then it could go in the %ProgramData% path.
That complicates thing though. If it does not matter if there are multiple copies of the file, then it is easiest to include it within the RBZ archive.