Chained Payments with Global Cart

Hi there,
I have installed Marketpress on a multi site installation. All seems ok at the moment. The only issue I have is that it seems I can only take a percentage of sales if I have paypal chained payments activated and this will then not allow me to have a global cart.

Surley this is vital to have a shared global cart for this sort of 'marketplace' like etsy or amazon. Has anyone figured out a work around as this is a bit of a problem for what I want to do.

The only other thing I can think of is for me to invoice people for commission on a monthly basis but will I get to see the value of sales each store receives on the network? I am happy to work this way around but would be amazing if it all happened automatically...

Surely someone has achieved this using Marketpress?

Thanks so much in advance