There's an option under Settings > Billing/Payment > Invoice Customization to show "Payments/Credits" on the invoice. When enabled, a Payments section appears (if there are any) on the PDF invoice, but not on the HTML invoice.
If enabled, also:
- Add the Payments section on the client HTML invoice
- Add the Payments section on the client "Pay" link (If a partial payment is applied to an invoice, the total shown reflects the difference but it's not apparent why it's less than the item amount.
See screenshots.