When you're trying to sell something to people, you can offer to "throw in" something extra. "Throwing in" something means to offer extra stuff without raising the price. For example:
If you buy two pairs at the regular price, we'll throw in a third pair for free.
What about if I throw in an extra battery and memory card?
People use this phrase when they're negotiating with someone one-on-one to sell something. You can also hear it on some TV commercials.