The phrase "deal sweetener" means an additional thing or benefit that is offered in a negotiation or agreement to make it more attractive or appealing to the other party involved. Full definition
Talk about something for everyone, with irresistible deal sweeteners everywhere. (pocketnow.com)
As deal sweeteners go, three months of Amazon Music Unlimited streaming instead of one isn't so remarkable. (pocketnow.com)
There isn't whole lot of leeway, but it doesn't hurt to ask for deal sweeteners — like more free copies of your book (they often throw in free copies of your book). (cnet.com)