Hi! I received my first STI kit.
I am fine with everything except, when I put everything back into the pre-paid envelope can I just put it into a post box?
Or do I have to physically go into the post office to send it off?
It’s bright yellow and has ‘biological substance’ written all over it.
I would be a bit embarrassed to go into the post office but I will do it if I need to ! 
Sorry if that’s a silly question I’m just not sure!
Thanks!