impulse buy
noun /ˈɪmpʌls baɪ/
  /ˈɪmpʌls baɪ/
 (also impulse purchase 
 /ˈɪmpʌls pɜːtʃəs/
  /ˈɪmpʌls pɜːrtʃəs/
)- something that you buy without planning to do so in advance, and without thinking about it carefully- It was an impulse buy.
- Vending machine purchases are often impulse buys.