Hey there, let’s unpack this step by step so you know exactly what to do:

Core Differences Between the Two Platforms #

• developer.paypal.com: This is your all-in-one developer command center. It’s where you’ll access every resource you need to build and test PayPal integrations—full API docs, SDKs, tools, and most importantly: managing your test applications and creating sandbox test accounts (both merchant and buyer profiles). Think of it as the "back office" for your development work.
• sandbox.paypal.com: This is the simulated end-user version of PayPal. It looks and acts just like the live PayPal site, but all transactions are fake (no real money changes hands). You’ll use the sandbox test accounts you created in the developer portal to log in here, test checkout flows, view merchant dashboards, and simulate customer payments. It’s the "user-facing" testing environment.

Your Specific Questions Answered #

Do I need a new Sandbox account, or can I use my existing developer account? #

No need for a separate sandbox account! Your existing PayPal developer account (the one you use to log into developer.paypal.com) lets you create as many sandbox test accounts as you need right inside the portal. These are the accounts you’ll actually use for testing—you’ll have a sandbox merchant account linked to your test app, and sandbox buyer accounts to mimic customer actions.

How to generate API credentials for Express Checkout SOAP API testing? #

1. Log into developer.paypal.com
2. Head to the My Apps & Credentials section
3. Double-check you’re in the Sandbox tab (not the Live tab—critical for testing!)
4. Either create a new app (label it something like "Express Checkout SOAP Test") or use an existing one if you already set it up
5. Under the app’s details, look for the Classic API Credentials section—this will give you the API Username, API Password, and Signature you need for the Express Checkout SOAP API. These are your sandbox-specific credentials, so they’ll only work with PayPal’s sandbox endpoints.

Which environment should I use for testing my application? #

• For your backend SOAP API calls (like setting up the Express Checkout session), send requests to PayPal’s sandbox SOAP endpoint: https://api-3t.sandbox.paypal.com/2.0/
• For any frontend parts where the user needs to log in and complete payment, redirect them to sandbox.paypal.com. Use one of your sandbox buyer accounts to go through the checkout flow just like a real customer would.

内容的提问来源于stack exchange，提问作者davioooh