Gratuities for other travel services, including but not limited to taxis and rideshare, are reimbursable at a rate not to exceed 20% of the cost of the service.
If people tip more than 20% their reimbursement will be capped at 20%.
If there is no receipt for the tip because it was paid in cash, it still possible to receive reimbursement if we are within the 20% limit. You can just put a comment that it was paid in cash and no receipt.