Aliexpress
Click here… to search for Peacefair PZEM-023 0-100V Voltage Tester on Aliexpress
Here are coupons that you can use.
Latest price update (UTC time):
Banggood
Banggood warehouse is located in Hong Kong. There is one coupon right now! Check this out.
Link: Banggood product page
Coupon (13.15 USD): BG00c906
(CouponsFromChina.com tip: paste coupon code in your cart and click Apply after clicking “Proceed to checkout”. Click to copy code and open shop)
Old coupons (from the oldest to the newest): BGRCParts8, BGJUNEGSG9, Cuponation9, 9OFFSUMMER
(you can check these old coupons out, but these probably won’t work)
Price history and price alert
Check out price history chart (for all/most countries) for Peacefair PZEM-023 0-100V Voltage Tester.
Price records source(s): Banggood
Click on image to enlarge. 
You can set price alert (for Banggood).