Step 1: Make sure your job has a checklist already and then click 'Invoice' to send the invoice to your customer.
Step 2: Click on the paper clip icon to add an attachment to the invoice.
Step 3: Select 'CHECKLISTS' then select the checklist you wish to attach to the invoice.
Step 4: Click on 'Attach'
Step 5: Continue to send the invoice like you normally would. You will notice that next to the paperclip icon you will see a number for how many attachments (photos and checklists) are attached to the invoice.