Each of these is a refusal this regime lets a counterparty attempt, paired with the answer to it. Reading them before you write is worth more than any amount of polish on the letter itself.
"You are not our customer — the sender is"
high likelihoodThe carrier declines to deal with the recipient of a delayed or lost parcel on the basis that its contract is with the shipper.
What answers it
Often correct, and it points at the better claim rather than closing one. If you bought the goods, your contract is with the SELLER, who is responsible to you for delivery until the goods are in your hands and who carries the risk of loss in transit under consumer law. Stop arguing with the carrier and put the claim to the seller — they are the ones who owe you the goods or the money, and they can take it up with the carrier themselves.
"The courier says it was delivered"
high likelihoodThe retailer relies on a delivery scan or a doorstep photograph.
What answers it
Risk does not pass until the goods are in your physical possession, and the burden of proving that is on the retailer. Ask for the proof of delivery — the signature, the photograph and the GPS data — and check whether it shows your address at all.
Consumer Rights Act 2015, s. 28 — s. 28 (delivery of goods) and s. 29 (passing of risk)
"The service guarantee is suspended"
high likelihoodThe carrier says its money-back or service guarantee does not currently apply to the service you used, or did not apply on the date you shipped.
What answers it
This can be true — the large carriers have suspended their guarantees for extended periods, and a suspension in force on your shipping date does defeat the guarantee claim. Two things to do rather than give up. First, ask them to identify the suspension notice in force on the date you shipped and the service it covered; suspensions are usually service-specific and the answer is often that yours was not covered. Second, remember the guarantee is only one route: a suspended contractual guarantee does not touch your rights against the SELLER under consumer law, which is where a late or missing online order is usually best pursued.