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!