It is not “stored” the addons register themself to be used as mailer. Of course, only one is used by Newsletter.
If you want to enable just one, automatically, the best option is to keep only one active at time, using for example wp-cli to deactivate and activate the respective plugins.
I tested the following wp-cli commands for the switch:
wp plugin activate newsletter-amazon
wp plugin deactivate newsletter-mailjet
and vice versa:
wp plugin activate newsletter-mailjet
wp plugin deactivate newsletter-amazon
This way, I had to enable the delivery addon first and then deactivate the plugin.