Shipping options for MarketPress

I'm trying to create some basic shipping options - but no matter how I try, they don't work. All of the information I find via the forums seems to pertain to older versions (pre v3) which are not working.

All I want is a user selectable shipping method - such as:

Next Day ( £10.95)
Next Day By Noon (£13.00)
Next Day 9.30am (£24.99)
Saturday (£21.00)

As MarketPress don't have a built-in option to this, I am creating this custom option. I can get the options to display on the shipping form - so they can select which one they want.. I just don't know how to read this back into MarketPress to add the required amount of shipping

This is the code I created:

https://github.com/symtal/shipping-rate/

Can you give me some pointers?

Also, how to make this a portable external plugin - so that it doesn't get wiped if MarketPress is updated.