Aliexpress
Click here… to search for 20L Waterproof Insulated Backpack 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 (22.35 USD): BGTY704CP
Coupon (22.35 USD): BG10OFFAFF
(CouponsFromChina.com tip: coupon code has to be applied in shop’s 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): BGYB659, BGOL786, BGZOB06, AFfbG10, BGAFF10OFF, bGhOnEy10, BGZPBC15, BGCuponation10
(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 20L Waterproof Insulated Backpack is here.
Price records source(s): Banggood
Click on image to enlarge. 
You can set price alert (for Banggood).