I totally understand what you mean.
The donating of the p&p is optional - it is up to the seller whether he / she wishes to do so. Some sellers go for donating the p&p as an extra, some include it in the bid price, and then we have others who are happy to donate items, but are not in a financial postition to donate the p&p (I was one of those autumn of last year) or perhaps as you mention, donate a heavy item with hefty postage and prefer not to donate the p&p.
This last one is one reaon why we try to steer away from having heavy items on the YDC because of the high p&p. Not because it has to be given to the charity, but it does tend to put some bidders off as they themselves prefer not to end up paying - for example - £5 p&p for a charity auction item where the final bid is something like 99p. If that makes sense.
I hope that helps.