Each retailer can have a private link that opens in their phone's browser. No app to install.
Step by step
Open the party
Go to Parties and open the retailer.
Share the portal link
Click Retailer portal → Share on WhatsApp. The link shows their ledger, open bills, a UPI button and your catalogue with their rates.
Switch it off any time
Turn the link off from the same place; it stops working at once.