What's with the tissue?

[ladyramkin2000]

No: HPFGUIDX 95048

Many thanks to all the people who answered my simple-minded question 
about the life-debt, especially Steve, for the sweet and comforting 
quotation.  Can you now answer another one? (As someone said 
recently, I am better at asking questions than answering them).
What exactly is going on with the "gifts" the Dursleys send Harry at 
Christmas e.g. a single tissue, a 50p coin? Why do they bother to 
send him anything?  Is it part of the contract Petunia has 
reluctantly agreed to, to protect Harry, that some token must be sent 
and is she sending the very least she can get away with?  Come to 
that, how do they actually send the tokens, since owls are not 
available to them, and the Royal Mail presumably doesn't deliver at
Hogwarts.  Sorry if this has been asked before, but that tissue is 
really worrying me!
Sylvia (who also wonders what would be the consequence of wilfully 
reneging on a life-debt.)