A surprise invoice

Short answer. Book the invoice on Add billing data after you confirm the billing period. That invoice is the total for the vendor and the period. Usage already measured stays attributed and is scaled to the invoice. Taxes, fees, seats, and credits stay on their own lines.

Confirm the period

Open Add billing data. Choose the vendor on the invoice.

Invoice date prefills the billing period as the calendar month before that date. The note says "The billing period below starts as the month before this date. You can change it." Set Billing period start and Billing period end to the period on the invoice, then check I confirm this billing period. Changing a date clears the confirmation.

The drop zone stays inactive until the period is confirmed. Without the check, the upload is refused with "Confirm the billing period before this invoice can be booked."

What the invoice does

The confirmed invoice is the total for that vendor and that period. Usage Meridian already measured in the period stays attributed to the same agents. Meridian scales that usage so the usage total matches the invoice.

Taxes, fees, seats, and credits are not part of that scale. Each one is its own line. A negative amount is a credit. That is why the invoice total and the usage total can differ by tax, a fee, a seat charge, or a credit.

A likely duplicate

If the vendor, the period, and the total match an upload already booked, the dialog title is Possible duplicate invoice. The message is "This looks like the same {vendor} invoice for {start} to {end} (${total}). Replace the existing upload, or add it as a separate invoice?"

Replace the existing upload replaces the earlier file. Add as separate books this file as another invoice.

A file that overlaps a connector is a different dialog. "Duplicates what the connector pulls" holds it as a control total. "Covers spend the connector misses" books only the gap. The amber note on the page says the invoice is the total for its confirmed billing period, usage already measured in that period is scaled to it, and a likely duplicate asks you to replace the existing upload or add it as a separate invoice.

After it books

Read Dashboard again for that vendor and period. The usage figure is the scaled invoice usage. Tax, fee, seat, and credit lines sit beside it. A Teams total for a closed month can move from Estimate to Settled once every row in that month is reconciled or invoiced.

The field-by-field page is Seat CSV and invoice uploads.