According to my DB the lowest price is 7.99 USD for MT01US Socket (in Banggood). You can create price alert here, copy coupons and view price history. Let’s see what best price is ATM.
Latest price update (UTC time):
Banggood (7.99 USD)
Banggood warehouse is located in China. Hmmm… There is no active coupon in Banggood at this moment. But wait a sec… Is it a good price? It looks like it’s a good price (lower than average (8.33 USD)).
Link: Banggood product page
Price: 7.99 USD
Exclusive Allowance Coupons: https://bit.ly/3xWgD9k
Old coupons (from the oldest to the newest): BGGines, BGM1TOOLS88, BG20236Tool, BGTOOS729
(probably won’t work, but you can try them out)
Aliexpress
Click here… to search for MUSTOOL MT01US Socket Tester US Plug on Aliexpress
Here are coupons that you can use.
Price history and price alert
Price history (for all/most countries) for MUSTOOL MT01US Socket Tester US Plug is here.
Price records source(s): Banggood
Click on image to enlarge. 
You can set price alert (for Banggood).