ARTERNAL doesn't have a merge button for invoices. Instead, you move everything onto one new invoice. Do the steps in order.
Before you start
Open the Payments tab on each old invoice. Write down every payment's amount and date.
Take the works off the old invoices
Open the old invoice and go to the Edit tab.
In the Items section, click the pencil icon on the work's line.
Click the trash icon next to Save. The line disappears and the total updates.
Repeat for every work, then do the same on the other invoice.
The old invoice now shows a total of $0. Its payments stay recorded on it. That is expected.
Archive the old invoices
Click the three-dot menu next to Download PDF.
Click Archive, then click OK.
You go back to the invoice list and see "Invoice archived".
Archive the other invoice the same way.
Important: remove the works before you archive. If an invoice still has works on it, ARTERNAL clears the invoice status on those works and deletes their costs, even if the works are also on another invoice.
Archived invoices are under Archived in the left menu of the Invoices page.
Create the new invoice
On the Invoices page, click Create Invoice and choose a template.
Under Bill To, click Search for Contact and pick the collector.
Under Items, click Select From Inventory. Search for each work and click it to add it.
Build the new invoice last. Adding the works to it puts their invoice status back. The new invoice gets its own invoice number.
Add the payments
Open the new invoice's Payments tab.
Click Add Payment. Once one payment exists, the button reads Add Partial Payment.
Enter the Payment Amount. Add a note in Payment Notes if you like, for example "Moved from INV-00059".
Set the Date to the date of the original payment, then click Save.
Repeat for each payment from the old invoices.
The Paid and Outstanding boxes at the top update.
❓ FAQs
Q: Can I merge two invoices?
A: Not with one click. Use the steps above to move the works and payments onto a new invoice.
Q: I already archived an invoice that still had works on it. What now?
A: Open it from Archived, click the three-dot menu, then click Unarchive and OK. The deleted costs come back. Then remove the works and archive it again.
Q: Why does the old invoice show $0 with a payment on it?
A: You removed its works, but the payments stay recorded on it. Archive it once the works are off.
Related: How to make a new invoice and Track expenses for an artwork.