Postmen is another API option.

Shippo appears easier to code for, I'm happy enough with it (although just reported some bugs).

We can do a very very basic implementation initially to solve the problem of Composr not having any way to have shipping cost vary by distance (a major issue I think)...

1) Have a config option specifying the density of items, so we can approximate the dimensions as a cube using the weight
2) Formalise the units -- allow the config option to have either pounds (plus inches) or kilos (plus metres)
3) Have a set of config option for shipping addresses. Must be able to cover the following fields: street1, city, state, zip, country, phone
4) Have a config option for the Shippo API key
5) Document; including suggestion to delete all but one shipping option in the Shippo account as only the first returned will be utilised
6) Error messaging if Shippo fails

Note for my reference - the customs object is not required for getting the rate quotes. We just need to send : object_purpose, address_from, address_to, parcel, async