Marketpress multisite - global shopping cart - one PayPal Express account

I'm trying to setup my Marketpress multisite website with a global shopping cart and PayPal express. At the beginning, I only have three sites in the multisite and I'm managing things for all of these site owners. I'd like all purchases from the three sites to come back to one PayPal account. Is this possible?

In Network > Settings > MarketPress I've put my PayPal API credentials and on each site in Products > Store Settings I've put the email address I used to setup the PayPal account. When I purchase a single item, it works perfectly. When I try and purchase two items from different stores, I get a 10415 error from PayPal. It actually buys one of the items, but not the other.

Grateful for any advice.

Thanks,
Peter.