What is PayPal Merchant ID? A PayPal Merchant ID (PayPal MID) is a unique 13-character combination of random letters and numbers. The PayPal MID is assigned to you automatically, once a PayPal business account is created.

Firstly, in your PayPal business account click on your name, then go to the Account settings section:
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Secondly, open the Business Information tab and copy the Paypal Merchant ID:

Thirdly, paste the PayPal Merchant ID into the Seller ID field of the payment gateway settings in the admin panel:

One of the frequently asked questions is why is a payment not auto approved, the other possible reason is connected with PayPal Instant Payment Notification settings or IPN.

Instant Payment Notification (IPN) is a message service that automatically notifies merchants of events related to PayPal transactions. Merchants can use it to automate back-office and administrative functions, including automatically fulfilling orders and providing customers with order status.

You need to enable IPN in the Account Settings > Website payments > Instant Payment Notifications:

Choose IPN settings and enable IPN messages specifying your dating website URL:

Instead of site.com, use your domain name and specify Notification URL:

https://site.com/payments/response/paypal

Visit Marketplace to find other payment gateways integrations.