Aliexpress
Click here… to search for Digoo DG-MC1 Solar Power Calculator on Aliexpress
Here are coupons that you can use.
Latest coupon and price update (UTC):
Banggood
Banggood warehouse is located in China. I am tired of searching coupons, but I found one for you…
Link: Banggood product page
Coupon (4.82 USD): AFfbG10
(CouponsFromChina.com tip: apply code in your basket (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): BGDGMC1
(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 Digoo DG-MC1 Solar Power Calculator.
Price records source(s): Banggood
Click on image to enlarge. 
You can set price alert (for Banggood).