In the preferences there is an option to choose whether pick slips will print by line number or by shelf location.
For either a pick slip or an invoice, print the items alphabetically you:
- Open the invoice and click the header for the items column
- Click the blue "recycle" icon at the bottom left corner of the list.
- You'll be asked if you want to renumber the lines. If you accept, lines will be renumbered to match the displayed order, and when you print the pick slip or invoice, it will print in alphabetical order.