packers are just flaccid penises that you "pack with" to make your clothes fit so that it looks like it is supposed to, kind of like a binder, just giving you the correct appearance. Some companies make "dual" packers that you supposedly can have sex with and pack every day(because no one wants to have an erection 24/7) but a normal packer you can not have penetrative sex with.
An STP(stand to pee) is just a packer with a device, usually tubing through the center of the packer with a medicine spoon for example that fits against what you have now so that you can pee standing up.
there are packers, STP's and, "strap ons" that you can glue on to you and there are all 3 that you don't have to glue on.