Just check coupons out (with price alert and history). What is best price?
Material: Aluminum Alloy, Color: Black, Size: 10.24×7.32×7.28″ (1cm=0.39Inches), Brand: Raitool, Model: MR01, Type: Ruler
Aliexpress
I could not find it on Aliexpress. Click here to search by yourself.
Here are coupons that you can use.
Latest coupon and price update (UTC):
Banggood
Banggood warehouse is located in China. We have found one coupon for this product!
Link: Banggood product page
1. ($14.01): BG589b2e
put item in your cart and apply coupon code there after clicking “Proceed to checkout”. Click to copy code and open shop.
Exclusive Allowance Coupons: https://bit.ly/3xWgD9k and https://couponsfromchina.com/short/5cd
There were country-specific coupon codes published on Couponsfromchina social channels and price alerts. Coupon price may vary by country, because there’re different VAT rates, shipping cost etc. There is a chance that some latest coupons are still working:
Old coupons (from the oldest to the newest): BGea4fb9, BGherra, BG00c906, BGGines, BGPTL088, BGHT15, BGPHOOL8, BGHPT15
(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 Raitool AR01 260x185x185mm Aluminum Alloy Triangle Ruler is here.
Price records source(s): Banggood
Click on image to enlarge. 
You can set price alert (for Banggood and Aliexpress (only if product name will be the same as on Banggood and I will post it on social media though, so it’s more like Banggood price alert)).