You can use Flaunter to help you manage this process with friendly reminder emails, including an invoice with outstanding samples.
Firstly, to send a reminder, you'll need to ensure you have entered the contact's email address when creating a send out.
To find outstanding send outs, select 'Sample Tracking' at the top of the dashboard.
Select 'Send outs' and use the dropdown on the top right-hand side to sort send outs by 'Overdue'. You can then free text search the contact's name or scroll down to browse.
To send a reminder email:
Select the white dots on the top right-hand side of the send out.
Select 'Edit/View':
Select 'Email invoice to contact' + enter in details for return!
