Ive come across this one myself.

I reckon this one is okay but its a bit rude - a bit of politeness and perhaps the offer to send a consolidated invoice automatically would have a better response. I never have problems with multiple purchases - if anything i have people who make a multiple payment and pay the full shipping regardless - when i tell them not to bother paying postage the next time they ignore me!

As i said before, i allow for the paypal fees, so if people pay separately that is fine. However, this is something you can discuss after the event. I have a number of customers who pay weekly, and others that warn me they are going to make anumber of purchases before settling up.