Signed for showind delivered buyer saying not

hi all,first time posting.
I sent a parcel 2nd class signed for within the dispatch time and it has been delivered within the estimate too.
The royal mail have scanned it and it shows delivered the time and signed for.
The buyer is claiming not received and has been sending me messages just 2 days after purchase even though i have 3 day dispatch and then the delivery time on top.
Can somebody please tell me what stance amazon will take on this.
many thanks