Flow of Work
This is an outline of the workflow. You can start using delivery options before the check-out, if you wish, but the most common place is at the end of the check-out process.
This document doesn't cover the integration with MetaPack Manager (MM) but in general terms, when you do a "create and allocate consignment" call in MM, you'll provide the booking code or service group which are provided in the MO response as well as the number and characteristics of the items the user confirmed as part of his shopping basket.
The booking code is carrier specific and the service group is more generic and only identifies the service type.