Receipt for payment of visa

On the 11th of December I left London to spend Christmas and New Year with my son in Australia. I took a two night stopover in Hanoi on way and three night stopover in Ho Chi Minh on my return. My travel agent told me that I did not require a visa as I was not staying in Vietnam for the required fifteen days. However, on my return, at check in at Sydney on 6th Jan for flight VN772 to Ho Chi Minh, I was informed that I did in fact require a visa. I was told that there was a clause stating that a visa is needed if you leave Vietnam but return within 31 days. I eventually managed to do this by paying 250 Australian Dollars in cash to a member of your staff there. By this time all check in desks had closed. The lady I gave the money to said she would sent the receipt details to Immigration and then they could send the visa to my phone via WhatsApp. While she did that, another staff member found me somewhere to check in when the visa came through on my phone. By this time my flight was already boarding, and in my panic to get my flight, I left without getting a copy of the recept for my money paid for the visa. I am now trying to make a claim on my travel company for the cost of the visa. Could you please send me a copy of my receipt, or some evidence that you actually received 250 dollars from me on that day. I have sent a WhatsApp message to the person who sent me the visa, but have had no response. I would really appreciate your help. Thank you. Marilyn Kirby.