Basically copy coupon, view price history or create e-mail price alert. What is the best price?
Aliexpress
There’re some similar (or not) items on Aliexpress
Click here to search by yourself. Here are coupons that you can use.
Latest price update (UTC time):
Constant Current 2Ax1 / 1Ax2 / 0.5Ax2, Operating Temperature 0-40℃, Size 110x53x30.5mm （LxWxH）, End-of Charge Current ≤150mA, End-of Charge Voltage 4.2±0.05V/1.45±0.1V, Brand XTAR, Weight 62g, Model FC2, Input DC 5V 2A
Banggood
Banggood warehouse is located in China.
Link: Banggood product page
1. ($7.99): BGFC22
2. ($19.30): BGTorch4
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): BGwhielabel, FIN0905, 4BLF, BG10OFFAFF1, BANGGOODSD
(system checks these coupons on every price check (like once per 24 hours), but you can check them out anyway)
Price history and price alert
Here is price history (for all/most countries) for XTAR FC2 Battery 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)).