Larger drawings, at least, are normally audited or even performed by an accounting firm, at least in the U.S., if nothing else for the protection of the sponsoring organization.

do the draw at a Charity dinner

As jtar said, first be sure that the organization has all of it's ducks in a row in regards to the applicable laws and such, but keep in mind that's their responsibility, not yours.

I'm not sure that having a computer draw the winner at the dinner is the best procedure for the anticipated anticipation and dramatic effect. I think I'd do it the old-fashioned way, print out all the receipts and what have you, put them into one of those turning drums and draw the winner by hand.

No matter WHAT system you set up on a computer to randomly select a winner, large sections of the audience will believe that the computer was rigged and nothing you say will persuade them otherwise.

Yeah, I suggested the printed receipts but apparently the Charity (whether they get it or not is another question) are after 30,000 donations over the next few months - so the guy I'm dealing with is against it because of the workload of that many entries.

The draw might take place in private now (the organiser keeps changing his mind) with a solicitor and independant judges as he is keen to do the draw electronically.

Any suggestions how it can be done using the PayPal payment receipts?

I suppose just selecting a receipt at random would suffice from his inbox?

This allows you to use any method of picking a winning number which might be conveniently and convincingly done in public at the banquet. i.e. pre-printed numbered tickets (make sure to get them already torn-apart!), etc.

The donation script can increment the serial number when the donation is made, and pass this along to PayPay in the item description. ("Donation - Lottery Entry #123").

Small wrinkle is payments that turn out not to go through. You can either assign a second number (when the receipt comes through) and record that in the database, picking from 1 to a smaller n, or simply hope that the winning # won't be an invalid one. If it does turn out to be invalid, you just pick another number, and it provides a bit of excitement to the crowd, and you can have a little pre-prepared missive about not following-through on pledges and how close somebody came to winning had they not welsh.... oh, this is in England, better not! :)

Once the winning number has been picked, all you have to do is locate the winning entry. If you have maintained a database of entries, this should be a snap, and can be displayed on a web page. Of course, you could simply do a search in your email client for the corresponding PayPal receipt.

I think a nice touch would be to have a suitably-gaudy web page complete with annoying alarm sounds, flashing lights, etc. that would go live as soon as the winning number is entered in via an admin page during the ceremony. You can dramatically announce that the winner will be known world-wide the second it is announced to the live crowd.

Put that page up on a projector, and you will have satisfied both the need to choose a winner via an easily understood, time-tested, auditable means, while at the same time providing the desired high-tech flash.

There now. I'd be happy to donate my fee back to the cause. ;)

talk to an english or welsh solicitor who is a specialist in the applicable laws ( scottish laws are not the same )...

rule 1
CYA
rule 2
do rule 1 before moving on ..;-)