This is a common requirement for Indian businesses with multiple branches under a single GSTIN. Here's the recommended Odoo approach:

Standard Odoo Setup (no custom module needed):

Step 1: Use Warehouses for branches

Create one Warehouse per branch (Inventory → Configuration → Warehouses). Each warehouse = one branch location. Products and stock will be tracked per branch.

Step 2: Keep one company (single GST)

Since all branches share one GST number, keep all branches under one Odoo company. Use branches/warehouses for stock separation — no need for multi-company.

Step 3: eCommerce branch selection

Standard Odoo eCommerce does not have native "branch selection" for customers on the website. However:

• You can use Product Availability by Warehouse (configure via routes/procurement rules)
• Add a Custom Attribute or Note on checkout for branch selection
• Use Multiple Websites (one per branch) where each website uses a different warehouse

Step 4: Stock visibility per branch

In Odoo 17+, you can configure which warehouse's stock is shown on the website via Website → Configuration → Settings → Inventory.

For a complete solution (customer picks branch on website, sees only that branch's stock, payment routed correctly), this requires a custom module. We can provide this for free — reach out at custom@trewac.com.