Tip / Gratuity Option at Payment

Add an optional tip/gratuity field on the payment page so customers can add a tip when paying an invoice. Ideally with preset percentage buttons (e.g. 10/15/20%) plus a custom amount, and a toggle so the business can turn it on or off per invoice or workspace-wide.

Especially valuable for restaurants, catering companies, event professionals, and other service-based businesses where tipping is common and currently there's no way to capture it through the invoice.