According to my DB the lowest price is $11.42 for 4.2A Waterproof Car 2 Port Dual USB Charger (in Banggood). You can create price alert here, copy coupons and view price history. Do you want to know what the best price is?
4.2A Waterproof Car 2 Port Dual USB Charger Socket Power Outlet with LED Voltmeter for 12-24V Car Boat Marine ATV Motorcycle – Blue
Latest coupon code price check (UTC time):
Banggood ($11.42)
Banggood warehouse is located in China. Is it a reasonable price for this item? From what I can see this price is higher than average price ($10.11). There is one coupon available! Pretty cool, huh?
Link: Banggood product page
Price without coupon: $12.69
1. ($11.42): BGAMMT03
paste coupon code in your cart and click Apply after clicking “Proceed to checkout”. Click to copy code and open shop)
Exclusive Allowance Coupons: https://bit.ly/3xWgD9k and https://couponsfromchina.com/short/5cd
Old coupons (from the oldest to the newest): BGSCM11, BGCAR639
(you can check these old coupons out, but these probably won’t work)
Aliexpress
There’re some similar (or not) items on Aliexpress
Click here to search by yourself. Here are coupons that you can use.
Price history and price alert
Check out price history chart (for all/most countries) for 4.2A Waterproof Car 2 Port Dual USB Charger.
Price records source(s): Banggood
Click on image to enlarge. 
You can set price alert (for Banggood and Aliexpress (only if product name will be the same as on Banggood and I will post it on social media though, so it’s more like Banggood price alert)).