You just edit the cvar according to your needs, and check in the right functions if its the value you want it to be. If it is, then proceed, if not, return plugin handled and print them something. (Cvars are enabled disabled by admins with amx_cvar, so if an admin does amx_cvar g_EnableShop 1 in console, he sets the value of the cvar to 1, and you check that if its 1 you can proceed.