Can I send just the excerpt of a post?
You can, but there is a tradeoff to consider
When sending full posts you'll get higher engagement and better conversations (because people can reply to the post notification to leave a comment).
When sending excerpts you'll get more pageviews but lower comment counts and less engagement.
Now that you know, here's how to do it
To send only excerpts of your posts visit Settings > Postmatic > Options and check the Send only the excerpt instead of the full post content checkbox.
When you publish a post only the excerpt will be sent, and users will not be prompted to reply with a comment. Commenting on an excerpt doesn't work that well.
When writing a new post you'll notice in the Postmatic Delivery metabox there is a checkbox to choose to send just the excerpt or the full post. This checkbox will be set to whatever you globally declared in options. You can adjust per post at the time of publishing.