Great question—let me break this down based on real-world usage of PayPal's Billing Plans & Agreements API:

Automatic Billing (Default Behavior) #

Once you've completed the full setup flow, PayPal will automatically charge the buyer's payment method (credit card/PayPal balance) on each billing cycle—no manual action required from you. Here's what triggers this:

• You create a billing plan with your desired cycle (monthly, in your case), pricing, and auto-billing rules enabled (this is the default setting).
• You generate a billing agreement and send it to the buyer, who completes the authorization step (either via PayPal login or credit card entry).
• Once the agreement is activated, PayPal handles all recurring charges on the scheduled dates until the agreement is canceled, expires, or hits a predefined end date.

When Manual Action Might Be Needed #

There are only a few edge cases where you'd need to intervene manually:

• Failed Payment Retries: If an automatic charge fails (e.g., expired credit card, insufficient funds), PayPal will retry the charge based on the retry rules you set in your billing plan. If retries fail, you may need to manually trigger a retry via the API (using the agreements/{agreement_id}/bill-balance endpoint) or reach out to the buyer to update their payment method.
• Ad-Hoc Charges: If you need to charge the buyer an extra amount outside of the regular billing cycle, you can manually initiate a one-time charge using the same agreement.
• Agreement Management: Pausing, resuming, or canceling an agreement requires manual API calls or actions through the PayPal Merchant Dashboard.

Key Note #

Double-check your billing plan settings when creating it—ensure the auto_bill_amount parameter is set to YES (this is default) to enable automatic recurring charges. If you accidentally set it to NO, you'd have to manually approve each charge, which is not ideal for a monthly subscription flow.

内容的提问来源于stack exchange，提问作者Axil