Empty promises and non delivery of service paid for

One of the main attractions of an online shopping platform is convenience. But you seem to lack that. And this is not the first time this has happened to me. I especially pay for guaranteed express delivery before 17h00 the next day but you fail to deliver on time as you promised me. You happily take my money for this "guaranteed" service but you fail to deliver that which I paid you for. This is unacceptable and poor service. I demand a refund since you did not hold up your end of the agreement and I paid for a guaranteed service I did not receive.