5V High Efficiency Waterproof Solar Panel (Banggood, Aliexpress: Short Circuit Current ≥ 560ma, Open Circuit Voltage ≥ 7.2v, Cell Conversion Efficiency ≥ 24%, Minimum Output Power ≥ 3.5w, Standard Working Voltage ≥ 6v, Standard Working Current ≥ 540ma (the best price with coupon)).
You can create price alert here, copy coupons and view price history. What is best price?
Aliexpress
There’re some similar (or not) items on Aliexpress
Click here to search by yourself. Here are coupons that you can use.
Latest coupon and price update (UTC):
Banggood
Banggood warehouse is located in China.
Link: Banggood product page
1. ($18.05): 8BGwbg2204
2. ($18.25): 2203ActiesNL
3. ($18.44): BG00c906
paste coupon code in your cart and click Apply after clicking “Proceed to checkout”. Click to copy code and open shop.
Exclusive Allowance Coupons: https://bit.ly/3xWgD9k and https://couponsfromchina.com/short/5cd
Old coupons (from the oldest to the newest): 00c906, ADMITAD2212, Terence, Tabby20
(there’s a little possibility that old coupon will work)
Price history and price alert
Here is price history (for all/most countries) for 5V High Efficiency Waterproof Solar Panel. The lowest price was: $17.66.
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)).