Marketpress global cart paypal express paypal chained payments

My current project spec is to have a maketplace that has one shopping cart (global cart) at checkout, collect a commission off each product in the cart, products provided by, shipped etc by multiple vendors and that the buyer would see it as ordering from one company with many different products.

Chained payments does not seem to have the option to use global cart feature but allows for a commission on each sale. The negative is that a buyer has to checkout in each store not in one cart.

Global cart has one choice of Paypal Express checkout and does not have a chained payment option in network store settings.

How can I operate a multisite global cart and collect a commission on all sales?

thank you