I just want to the Ye Olde Commons site to pay for the meal plan, but how do I let them know which event I'm paying for or prove I've paid once there?

When it takes you to the Paypal site, there's a description box. List the name(s) of whoever you are covering and the date of the event.

Harald is great about sending confirmation e-mails.

There wasn't when I did it, I have my paypal receipt, but it doesn’t specify dates or event.

I suggest forwarding the paypal receipt to Harald (harald@yeoldecommons.com) with the details of the event, your name, and your character's name.

You can also pay per meal when you get there, or at least that's what I do at other games.