Proof of Posting slip

When we are appealing a late delivery defect why do ebay not accept a proof of posting as evidence that we have posted the item on time? 

It literally states on it that the item 'has been accepted here today' It has the time & date and the barcode number. So why do ebay not accept this as proof? 
I cannot make a claim for a lost item with Royal Mail without one but ebay do not accept them as evidence that I posted on time. 
A ridiculous situation