Carriers need Pro ($199 a month) to set this up; upgrade on Settings > Plan & billing. Fuel buyers get it free.

What syncs

A customer approves a record: an invoice to them in your books for exactly the approved amount, dated the day it was approved, carrying the record number. The customer is found by name or created the first time (NetSuite and SAP Business One bill it as the item you choose).
A payment through Demurly (card or bank), or an outside payment you confirm: a payment recorded against the invoice.
You record the payment in your books: within about 10 minutes the record shows as paid (the paid-in-your-books check). No need to confirm it again in Demurly.
On the record: "In QuickBooks as Invoice 1042", ", paid" once it's paid there.
When your customer also syncs (their bills, free for them), what they pay in their books shows as paid outside Demurly, for you to confirm when the money arrives.

Set it up

Settings > Integrations > Connect your system. One system invoices your records at a time.
NetSuite: enter the Item internal ID (the service item demurrage is billed as). SAP Business One: the Item code and the Payment account.
Records approved before you connected: Invoice them on the sync card.
Turn off Invoice approved records automatically to stop new invoices; payments still sync for invoices already sent.

SAP S/4HANA connects for a connection check only; send its invoices by EDI 210 or CSV.
Invoices you change or delete in your books aren't changed in Demurly.

"… totals $…, not the approved $…": tax or a line was added in your books. Fix the invoice there, then Retry.
"Fill in … before invoices can be sent": enter the invoice settings on the card.
A failed job shows its reason under Recent activity and on the record, with Retry.