When you set up labels, you set them up like this.
City, Stateá PostalCode
All of those are mail merge fields.....EXCEPT the comma.
So when you print out 17 labels on your 30-label sheet, you get a comma on EVERY label in the sheet. This is annoying.
To get around this, add a field in your data source, like your spreadsheet, that's just a comma. Make a new column anywhere, call it Comma, and type a comma in every row or record.
Then instead of typing the comma, just insert the Comma field.