UPDATE 2025

In general, Paddle, In production & sandbox environment allows only public access domain name, aka by typing : localhost -> WONT WORK.

How did I solve it? in short I used Ngrok.

HOW-TO:

step one install ngrok ( I am using Mac):

 brew install ngrok

run ngrok the <> (3000) should be the localhost:<> u are using

ngrok http 3000

Next, now ngrok will bring you a public domain for you app:

<<somthing>>.ngrok-free.app

go in sandbox dashboard to website approval (under checkout) and add this domain.

last step but dont forget it! (I was 1 hour searching for it), go to checkout (same as the one before and choose , checkout settings and add the domain name under : Default payment link

that's it!

this is how you can run "locally" sandbox and check full flow of transactions.