feat: [E-Commerce] Integration: PDF invoice generation

PDFInvoiceGeneration.md

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+# PDF Invoice Generation
+
+This module provides PDF invoice generation functionality for the e-commerce system, allowing orders to be converted to professional-looking PDF invoices.
+
+## Components
+
+### PDFInvoiceGenerator Class
+The main invoice generation engine that:
+- Creates professional invoice layouts in HTML
+- Converts HTML to PDF using DomPDF (if available) or falls back to HTML
+- Includes all relevant order information:
+  - Company details
+  - Customer information
+  - Order details and items
+  - Shipping information
+  - Detailed totals (subtotal, tax, shipping, discounts, grand total)
+  - Invoice number and dates
+
+### Order Model Integration
+The Order model has been enhanced with:
+- `generateInvoice()` method to create PDF invoices
+- `getInvoiceLink()` method to retrieve download links
+- `getInvoiceFilename()` method to get appropriate filenames
+
+### Invoice Controller
+A dedicated controller to handle invoice downloads at `/order/invoice/{order-id}`
+
+## Features
+
+### Professional Layout
+- Clean, business-appropriate design
+- Organized sections for all invoice information
+- Proper alignment and spacing
+- Print-ready formatting
+
+### Complete Information
+- Company header with contact information
+- Customer billing and shipping details
+- Detailed order items with quantities and prices
+- Complete breakdown of costs (subtotal, tax, shipping, discounts)
+- Grand total prominently displayed
+
+### Flexible Output Modes
+- Download PDF directly
+- View PDF inline in browser
+- Generate PDF string for custom handling
+
+### Fallback Mechanism
+- Uses DomPDF if available for proper PDF generation
+- Falls back to HTML with print suggestions if library missing
+
+## Usage
+
+### In Templates
+```ss
+<a href="$InvoiceLink" class="btn btn-primary">Download Invoice</a>
+```
+
+### In Code
+```php
+// Generate and download invoice
+$order->generateInvoice('download');
+
+// Generate invoice to view inline
+$order->generateInvoice('inline');
+
+// Get invoice download link
+$link = $order->getInvoiceLink();
+```
+
+### Direct Access
+Invoices can be accessed directly via:
+```
+/order/invoice/{order-id}
+```
+
+## Requirements
+
+The system works best with the DomPDF library installed:
+```
+composer require dompdf/dompdf
+```
+
+If DomPDF is not available, the system will generate HTML that can be printed to PDF.
+
+## Configuration
+
+The system is configured via YAML:
+- Routes for invoice access
+- Default settings for PDF generation
+- Integration with Order model
+
+## Integration Points
+
+The PDF invoice system integrates seamlessly with:
+- The existing Order model
+- The e-commerce workflow
+- The CMS for administrative access
+- Customer-facing pages for download access
+
+This provides a complete solution for generating professional invoices that meet business requirements while maintaining integration with the existing e-commerce system.

app/_config/pdf_invoice.yml

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+---
+Name: pdf-invoice-config
+---
+# Register the PDF invoice functionality
+
+# Add invoice extension to Order model
+Order:
+  extensions:
+    - OrderInvoiceExtension
+
+# Configure routes for invoice downloads
+---
+Name: invoice-routes
+After: 
+  - '#coreroutes'
+  - '#modelascontrollerroutes'
+---
+SilverStripe\Control\Director:
+  rules:
+    'order/invoice//$ID': 'InvoiceController'
+
+# Define required dependencies
+---
+Name: dompdf-defaults
+---
+# Default settings for DomPDF if installed
+Dompdf\Dompdf:
+  options:
+    default_font: 'Arial'
+    isRemoteEnabled: true
+    isJavascriptEnabled: true