Fetching details from the dictionary...

something thrown in (with a purchase); something extra; free gift; freebie; giveaway; premium; prize
(giving a) discount; price reduction
addition; embellishment (e.g. to a story)
お菓子を買ったらおまけがついてきた。
I got a bonus when I bought snacks.
そのうえおまけに天気まで悪くなってきた。
On top of that, the weather got worse too.
おまけをもらえると子供は喜ぶものだ。
Children are happy when they get freebies.