Guests can easily add additional gratuity to their proposal when paying their final balance.
In order for your guest to be able to add additional gratuity to their proposal, ensure that the system Gratuity is toggled on. To do this, head into Settings > Taxes & Fees, then click the "Enable" checkbox.
If a guest adding additional gratuity is a one-off case, you can always enable gratuity for that specific event. To do this, head into the event, click Edit next to Totals, then click into the checkbox next to Gratuity.
Once Gratuity is enabled, you're ready to send the proposal with the action of, "Pay Remaining Balance"!
The guest will be prompted to enter their card, as well as select additional gratuity.