According to my experience, it is cheaper to book directly from the airline. You can check the price online. No need to go through a travel agent.
If you choose to pay the maid cash instead of air ticket, I usually base on the fare of the 14th day after the last day of employment( they can stay 14 days more after finishing the contract). You can find the fare online and I will print it out and get your maid to sign on the print out to proof that she have acknowledged the amount for the airfare.
If request cash.
My fire helper also request cash because of she want to find another new job and stay at HK. So, I pay cash / cheque for the agency and let's agency to contact your helper.
Or
Pay cash to her but sign letter at agency will be better. Or send cash to agency for her to sign back with receipt.