I have just checked coupons. The lowest price I could get is 13.99 USD for Charging 10W Grinding (in Banggood). Just check coupons out (with price alert and history). Do you want to know what the best price is?
Latest price update (UTC time):
Banggood (13.99 USD)
Banggood warehouse is located in China. I have no active coupons for you right now. The question is – is it a good price? Well, this price is higher than average price (13.49 USD)
Link: Banggood product page
Price: 13.99 USD
Exclusive Allowance Coupons: https://bit.ly/3xWgD9k
Old coupons (from the oldest to the newest): BG589b2e, BGPT07
(you can check these old coupons out, but these probably won’t work)
Aliexpress
Click here… to search for USB Charging 10W Grinding Machine on Aliexpress
Here are coupons that you can use.
Price history and price alert
Here is price history (for all/most countries) for USB Charging 10W Grinding Machine.
Price records source(s): Banggood
Click on image to enlarge. 
You can set price alert (for Banggood).