Lining material 100% polyester, Material 100% polyester, Filler Polyethylene foamed cotton, Size 275x 150x 430mm / 10.83x 5.91x 16.93inch.
Just check coupons out (with price alert and history). What is the best price?
Aliexpress
There’re some similar (or not) items on Aliexpress
You can use link bot to get even lower price:
Click here to search by yourself. Here are coupons that you can use.
Latest price update (UTC time):
Banggood
Banggood warehouse is located in China. There is one coupon available! Pretty cool, huh?
Link: Banggood product page
1. ($44.80): BGTY704CP
this code has to be applied in your cart after clicking “Proceed to checkout”. Click to copy code and open shop.
Additional Banggood coupons: first page and second page
Old coupons (from the oldest to the newest): BGXM19889, AFfbG10, BGcamp305, BGDD23, BG13thMI20, BGXP2299, BGDQ15, BGB11D19, BGCPPACK20, BGH5889, bGhOnEy10, BGHALLOWEEN, BgcLuBofErtA10, BGZPBC15, BGCuponation10
(these codes were not working in latest coupon update, but you can check them out anyway)
Price history and price alert
Check out price history chart (for all/most countries) for Xiaomi 20L Backpack. The lowest price was: $21.99.
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)).