‘Free extras’ are things you would usually have to pay for when you buy a car, but the seller (or, dealer) added them at no cost to the buyer.  A sweet deal means a ‘good’ deal.  It means, ‘that is exceptionally good’ or ‘that is great’. 

你買新車得到這麼多配件贈品，真是划算！

Free extras 指「免費的附加物」。這些原本件件要錢的，車商（賣方）卻分文不取、大方附贈，對購車者（買方）而言，當然是 “a sweet deal” 一筆很棒的交易囉！