Color Black, Size Vacuum Stopper: 40x 70mm, Outer Material Stainless Steel, Weight 20g, Inner Material Silicone.
Basically copy coupon, view price history or create e-mail price alert. 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.
Link: Banggood product page
1. ($13.95): Terence
2. ($14.10): BG00c906
just apply this code 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): BGMS79K, AFfbG10, BGVDay10%off, BGHOMEG12, BGstpp25, 13f142, BGAFF10OFF, bGhOnEy10, BGdragonBoat, BGSS10%SW, bgshOOp10, BGHome15%, BGHOMEA15%, BGNOFR185, BgcLuBofErtA10, BGJATD185, BGCuponation10, BGHONEY2110, BG10OFFAFF, BGwhielabel, BGZXJU185, BG10OFFAFF1, BANGGOODSD, BGAlerabat1
(you can check these old coupons out, but these probably won’t work)
Price history and price alert
Price history (for all/most countries) for Xiaomi HUOHOU Vacuum Bottle Stopper is here. The lowest price was: $6.99.
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)).