More invoicing control

I would like to submit a feature request that would greatly improve the invoicing functionality of the plugin. These changes would provide more control and flexibility for businesses managing invoices and accounting.

1. Custom Invoice Numbering
   Ability to fully customize invoice numbering, including:
   Option to set a starting number for invoices.
   Support for custom prefixes, suffixes, and formats (e.g., “INV-2025-001”, “M-1001”, etc.).
   Option to manually override invoice numbers in specific cases (e.g., special customer requirements).

2. Separate Invoice Numbers for Test Payments
   Ability to assign a different numbering scheme for test payments or sandbox transactions.
   This feature would help clearly differentiate real invoices from test invoices, ensuring better financial clarity.

3. Credit Note (Refund Invoice) Creation
   Ability to automatically or manually create credit notes or refund invoices when transactions are refunded or canceled.
   Support for linking the original invoice to the corresponding credit note, ensuring accurate financial records.

4. Bulk PDF Invoice Export
   Ability to bulk export invoices as PDF files for easy processing or submission to accountants.
   Options to filter invoices by criteria such as date range, membership type, or payment status.
   Option to download all PDFs in a single compressed ZIP file for convenience.

These features would improve invoicing control, especially for businesses with more complex financial requirements. I believe they would significantly enhance the overall functionality of your plugin.