Query emails long reply response

I have a prepaid account with The Courier Guy and want to express my disappointment with the reply to queries from the PE Branch. I had a query on 31 May 2019 to find out where a parcel was. I emailed at 13.37pm to which some staff read my email, I only received a reply at 17.21pm. I only work until 17h00. Today 5 Jun, I had another query I emailed 15.37, my email was read and it is now 16.35 no reply. As a prepaid customer it was advised no telephonic queries but all queries must be emailed. How can I run a company where there is no proper lead time to reply on queries???? Very Disappointed customer