What should my "Thank you for placing an order" email look like?
Keep it simple. If you have not spent much time on customer appreciation, don't stress too much.
It can look something like this:
Dear XXX Customer,
I saw that you recently placed an order for your event and I wanted to say thank you for working with us. I hope your 'X' event went well (get personal here so the "thank you" feels genuine). It was a pleasure decorating for you. Please let me know if there is anything we can do for you in the future!