I have checked all old coupons and price is $7.22 for 5V 400mA Solar Panel 2W (in Banggood). Go down and check coupons out. Remember to check price history if it is available. What is best price?
Working voltage: 5V, Working current: 0-400mA, Process: EVA lamination, Output power: 2W, Output port: USB, Product size: 157 * 94mm, Wafer: class A polysilicon, Conversion efficiency: 18.8%, USB cable length: 50cm
Latest price update (UTC time):
Banggood ($7.22)
Banggood warehouse is located in China. There are no coupons for this product in Banggood. Price? Looks like a good price to me (average is $7.30).
Link: Banggood product page
Price: $7.22
Exclusive Allowance Coupons: https://bit.ly/3xWgD9k and https://couponsfromchina.com/short/5cd
Old coupons (from the oldest to the newest): BG589b2e
(probably won’t work, but you can try them out)
Aliexpress
There’re some similar (or not) items on Aliexpress
Click here to search by yourself. Here are coupons that you can use.
Price history and price alert
Price history (for all/most countries) for 5V 400mA Solar Panel 2W is here.
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)).