How can I choose which rows to merge?

From the Google Docs Add-on

Create a column in your sheet and name it whatever you want. Say Filter Rows to Merge. Then type 'x' (or any other text ) for the rows you wish to exclude from merging.

Then open the Options window by clicking the link from the sidebar and set the Filter Rows to Merge column in the Merge Specific Rows option. Click save to apply changes.

When you merge to emails or letters, rows with any value in Filter Rows to Merge will be excluded from merging. See image below for an example.

From the Google Sheets Add-on

A column called Filter Rows to Merge will be automatically created for the sheet you want to merge data from. Then type 'x' (or any other text ) for the rows you wish to exclude from merging. As shown in the image below, you can even create a formula to determine which rows should be excluded. In the example below it is excluding rows for employees who's gender is equal to "F".