Buyer asked for changes through message days after the order is marked as complete


This is a very awkward situation for me. A buyer requested a GIG 18 days earlier, I submitted the work prototype and haven’t heard from him since then. However i asked him several times about the missing details, however, he didn’t replied, he even marked a revision of the work prototype and based on those instructions, i submitted the work again. Although, the normal time to ask for revision or to review the work is 3 days. However the client didn’t replied within that time and fiverr automatically marked the order as complete. Now after 15 days past the review date, He is inquiring about “how we are doing on the receipt redesign”.
Although, i have never faced this issue in the past, still i have to confirm with Fiverr representatives what should i do regarding the project.
More over, he also messaged me that "Ok that is very interesting. If there is a problem on your side we will accept full refund with no negative comments. Thank you"
please guide me what should be my reply ?
and how should i deal with this client