Aliexpress
Click here… to search for UNIT UT658 USB Tester on Aliexpress
Here are coupons that you can use.
Latest price update (UTC time):
Banggood
Banggood warehouse is located in China.
Link: Banggood product page
Coupon (5.78 USD): AFfbG10
Coupon (5.97 USD): aff7off
(CouponsFromChina.com tip: just apply this code in your cart after clicking “Proceed to checkout”. Click to copy code and open shop)
Exclusive Allowance Coupons: https://bit.ly/3xWgD9k
Old coupons (from the oldest to the newest): 502C6B, e2bcea, TTxBG10, bgssaff9, BGBFAFF2019, BGCITYADS2019, BGVDay10%off
(sometimes old coupons work, so you can check them out (this site checks them on every price update))
Price history and price alert
Price history (for all/most countries) for UNIT UT658 USB Tester is here.
Price records source(s): Banggood
Click on image to enlarge. 
You can set price alert (for Banggood).