When I was updating this page the price was $27.89 for 5V 15W USB Solar Panel (in Banggood). Basically copy coupon, view price history or create e-mail price alert. Best price?
Latest price update (UTC time):
Process Laminated Sanding Board, Packaging Box Specifications 200*140*40mm, Current 0-2.4A, Conversion Rate 19%, Folding Size 170 * 120 * 30mm, Weight 400g（With Packaging Box）, Product Name 15W Solar Foldable Panel, Waterproof Grade IPX6, Output Voltage 5V, Unfolded Size 790 * 165mm, Output Interface Dual USB, Chip A-grade Polycrystalline Silicon
Banggood ($27.89)
Banggood warehouse is located in China. Too bad there’s no coupon right now in Banggood. What about the price? From what I can see this price is higher than average price ($26.07).
Link: Banggood product page
Price: $27.89
Exclusive Allowance Coupons: https://bit.ly/3xWgD9k and https://couponsfromchina.com/short/5cd
Old coupons (from the oldest to the newest): BG589b2e
(these coupons are old, so probably won’t work)
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
Do you need price history chart (for all/most countries) for 5V 15W USB Solar Panel? There we go.
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)).