Props: up to 3.5 inch, Bottom Plate: 2mm, Arm: 4 mm, Fit for Camera: 19x19mm, Mounting up front: 25.5×25.5 or 20x20mm, Top Plate: 2mm, Fit for：DJI Caddx Vista, Middle Plate: 2mm, Item Name: Chris Rosser AOS 3.5inch Frame kit, Mounting in the rear: 20x20mm, Weight: 49g (with all hardware).
Just check coupons out (with price alert and history). You probably want to know 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 coupon code price check (UTC time):
Banggood
Banggood warehouse is located in China. One coupon is available. Cool!
Link: Banggood product page
1. ($58.87): BGMatt2022
apply it 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): BGRCParts8, Tabby20, BG1efeb0, MVC8SPRING, 8BGwbg2204, tRaDetRaCKeR, 8wEbgaInS5, pROMo8cuPoN, BG8RCToy, hoNEYbGdeAlS, BGRCT10, BGJUNEGSG9, BGToys8, BG8OFFCOUPON
(I don’t think these’re gonna work, but you can try ’em out anyway)
Price history and price alert
Check out price history chart (for all/most countries) for Chris Rosser AOS 3.5 Inch Frame Kit. The lowest price was: $57.59.
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)).