GNB 3S 300mah, GNB 2S 300mah, Item name: Mobula7 V3 frame kit, Compatible Battery: Happymodel 2S 300mah, Usage: Spare Part for Mobula7/Mobula7 HD/Mobula7 V2 RC Drone, Weight: 5.5g, Wheelbase: 75mm, Brand name: Happymodel.
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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 coupon code price check (UTC time):
Banggood
Banggood warehouse is located in China. One coupon is available. Cool!
Link: Banggood product page
1. ($10.57): BGMatt2022
you should apply the coupon 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): MVC8SPRING, 8BGwbg2204, tRaDetRaCKeR, 8wEbgaInS5, pROMo8cuPoN, BG8RCToy, BGToys1, BGRCT10, BGJUNEGSG9, BGRCParts8, Cuponation9, 9OFFSUMMER, BGToys8
(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 Happymodel Mobula7/Mobula7 HD/Mobula7 V2 75mm V3 RC Frame Kit. The lowest price was: $10.39.
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)).