If you use Shopify, it is as easy as it could possibly be to add their Messenger bot offering. Just click “+” beside “Sales Channels” and add Messenger. You may be disappointed with the actual behaviour however. The bot only responds to users’ actions on “quick response” buttons - it seems that if they try to enter any other query, it is just ignored. A user is expected to make use of the default menu to interact with the bot.
Once you’ve added the Messenger sales channel, there are some customisations you can make - you can change the default message and the name of the various menu items, for example. These customisation options are extremely limited, however, and there is little scope for changing the behaviour of your bot.