PaddlePress Pro uses Paddle API to fetching products and integrates over webhooks. You can setup a sandbox environment to test integration.
Create a Sandbox Account
Create the sandbox account from – https://sandbox-vendors.paddle.com/signup (You don’t need to verify website address to test with sandbox)
Enable Sandbox Mode with PaddlePress

Enter the Sandbox credentials and then your website is ready to test with sandbox data.
Alternative Method
Alternatively, you can access webhook simulator on your paddle dashboard. Developer Tools > Alerts/Webhooks

Use https://example.org/?paddlepress-listener=paddle as payload URL and test subscription and one-off purchase-related actions.
Plugin recommendations for testing
Query Monitor: It is the developer tools panel for WordPress
Email Log: It allows log and view all emails that were sent from WordPress. So, you can preview the emails that your customers will get.
User Switching: It’s very handy to switch user and use “my-accounts” page as user.
Test checkout integration
Follow-up Paddle’s testing guidance for testing the checkout process.