Alternative: Augment your table by adding a "selected" field (Yes/No). Make a query that uses the [Selected] field in a WHERE clause but not in the SELECT clause. (On the grid, clear the X in the box that shows you wanted to see that field in the resulting data grid.) Then build your MailMerge on the query, not the table itself.
This step requires less VBA code though it is clunky and not well-suited to sharing. Nevertheless it works fine when you use it.