Are freebies completely free?

I ordered a couple of freebies and they came at the post office. so i went and got them and they were for free but are they for free? are they not gonna charge me after words like send me a bill or somthing? please help im really scared.... and the freebie i recieved today was from this animal package it comes with books, posters and smaller posters so its like a clroom set. Please help me are they free to keep