Multiple Shipping Price Options

Hi all,

I'm working on building a custom Shipping Plugin for MarketPress. I just came across an issue and was wondering if there is any way around it. The problem is that I would like to return multiple shipping rates for the user to select. For instance, I would like to give them an option for standard delivery and rush delivery. The "calculate_shipping" method in the class that extends the "MP_Shipping_API" class is supposed to return a string. How would I go about enabling multiple shipping choices? I'm assuming that this is not supported, but thought I would ask in case there is a way.

My option at this point would be to change the MarketPress core, but that is never a preferred option. Will you be adding this support? I can see that this is a necessary component in order for the USPS and FedEx (among others) shipping modules to be built. Any plans in the works?