Fix submit method

### What is the problem / feature ?

- The `AbstractOrder` model does not have the  `shipping_charge` property.
- Basket line model and order line model are different models, and `order` `AbstractLine` does not have the `line_price_excl_tax_incl_discounts` property.
- `order.shipping_method` can be a `django.utils.functional.__proxy__` object.

### How did it get fixed / implemented ?

- use `shipping_excl_tax` instead of `shipping_charge` on the order model
- get the `Amount` from `line.line_price_excl_tax` if the line is an `OrderLine`
- call `unicode()` with `order.shipping_method` as argument

*Here is a cute animal picture for your troubles...*
![image](https://cloud.githubusercontent.com/assets/765961/5495500/80636f7a-86bb-11e4-9616-af90d674797a.jpg)