One document, four variants
Quotation, order, invoice and credit note inherit from the same parent type. The line items with quantity, price and tax rate hang off the document, and the portal does the sums.
A number range per year
Invoices are numbered as RE-YYYY-NNNN, consecutively per year and per tenant. The next number is derived from the existing records, not from a counter that can slip.
Deadlines per document type
The quotation has a valid-until, the order a delivery date, the invoice a due date and a paid-on date. A traffic light shows which of those is being missed.
Drafts go, documents stay
As long as a document is a draft, it can be deleted. Once issued, it stays on record even if it is taken back. That is what the credit note is for.
Dunning in three stages
A payment reminder with no fee, a first dunning letter at 5 euros, a second at 10 euros. Seven days between stages, and after the second letter the run stops. What happens next is a decision for a person.
Partial payments are normal
Incoming payments are records of their own, attached to the document. Two partial payments against one invoice are the rule, not the exception, and receivables are calculated from them rather than from a tick box.