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Why Are My Invoice Numbers Skipping or Duplicating? A Troubleshooting Guide

22 threads Sep 23, 2025 Plugin → Pdf invoices & packing slips for woocommerce

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If you're using the 'PDF Invoices & Packing Slips for WooCommerce' plugin and have noticed gaps in your invoice numbering sequence or duplicate numbers being assigned, you're not alone. This is a common concern, especially for businesses that need strict sequential numbering for accounting or legal compliance. This guide will explain the most common causes and provide steps to diagnose and fix the issue.

Why Invoice Numbers Skip or Duplicate

Sequential numbering is the default behavior of the plugin, but several factors can interfere with it. Based on community reports, here are the primary culprits:

  • Race Conditions with Payment Gateways: This is a frequent cause. Some payment gateways, like specific PayPal configurations, can send two payment confirmations (PDT and IPN) for the same order almost simultaneously. This creates a "race condition" where two parallel processes try to generate an invoice for the same order. One process might "win," assigning an invoice number, but the other process can also trigger, causing a duplicate entry or consuming the next number without generating a visible invoice, thus creating a gap. Threads #2, #5, #6, #10, and #13 highlight this issue.
  • Order Status Changes: The plugin assigns an invoice number when the document is created, not when the order is placed. If your settings only create invoices when an order reaches "Processing" or "Completed" status, the sequence of invoice numbers will match the sequence in which orders are paid or completed, not the order in which they were received. If an older order is completed after a newer one, it will receive a later invoice number, making the sequence appear non-sequential when viewed by order date. This is normal behavior, as mentioned in Thread #20.
  • Free Orders: By default, the plugin does not generate invoices for orders with a total of $0.00. If these orders are interspersed with paid orders, they will not consume an invoice number, which can also make the sequence appear to jump. You can change this setting under PDF Invoices > Documents > Invoice > Disable for free orders (Thread #7).
  • Manual Intervention or Configuration Errors: Accidentally changing the "Next invoice number" setting, manually editing an invoice number incorrectly (e.g., including the prefix/padding in the number field as in Thread #21), or restoring a database backup can lead to large gaps or numbering errors (Thread #11, #15).
  • Deprecated Code in Custom Templates: If you use a custom template or another plugin that extends the PDF invoices, it might be using old function calls like invoice_number() instead of the updated number() method. This can cause errors and blank invoice numbers, as seen in Threads #14 and #19.

How to Troubleshoot and Fix Skipped Invoice Numbers

Step 1: Identify the Pattern
Check the invoice number log under WooCommerce > PDF Invoices > Advanced > Numbering. Look at the orders with skipped numbers. Do they share a common payment method (like PayPal or Razorpay)? Were they created around the same time? This can point to a race condition.

Step 2: Check Your Payment Gateway Configuration
If you suspect a race condition, review your payment gateway settings. For PayPal, a common fix is to ensure that only one payment notification method (either PDT or IPN) is enabled, not both, to prevent duplicate notifications. Consult your payment gateway's documentation for best practices.

Step 3: Review Your Document Creation Settings
Go to PDF Invoices > Documents > Invoice and check the "Attach to" setting. Invoices are generated when one of these emails is sent. If you need invoices created immediately upon order placement (regardless of payment), you may need a feature available in the Professional extension. Also, verify the setting for free orders if gaps correspond to zero-value orders.

Step 4: Check for Custom Code or Templates
Temporarily switch to the default "Simple" template under PDF Invoices > General > Choose a template. If the problem disappears, the issue lies in your custom template. Update any deprecated functions like invoice_number() to number().

Step 5: Correct Individual Invoice Numbers (Advanced)
For minor gaps, you can manually edit an invoice number. Open the order, find the "PDF document data" section, click "Edit" for the invoice, and enter only the raw number (e.g., 218, not 218-041023). Saving will reformat it correctly. For bulk corrections, the plugin's 'Number Tools' can be used with extreme caution and only after a full site backup, as changes are irreversible.

Step 6: Enable Debug Logging
If the issue persists, enable debug logging. The 'PDF Invoices & Packing Slips for WooCommerce' team provides a guide on where to find error logs. This can capture underlying errors during invoice generation.

Conclusion

Invoice number skipping is often a side effect of how orders are processed rather than a fundamental bug in the plugin. By understanding the interaction between payment confirmations, order statuses, and plugin settings, you can usually identify the root cause. Always test changes on a staging site first and maintain regular backups, especially when using advanced tools to renumber invoices.

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