Each document answers a different question. While the chain is intact, disputes are settled by paper; when a link is missing, they are settled by whoever remembers more loudly.
The order
The quotation — what, for how much, on what terms. The estimate — what the price is made of, attached to the quotation. The contract, where the estimate becomes binding as an annex. The handover act, confirming the work is complete and accepted and usually starting the warranty period. The invoice, referring to the contract and the act.
Every document refers to the one before it — the contract cites the estimate, the act cites the contract, the invoice cites both. That single discipline prevents most disputes.
Changes during the works
Record them as an addendum with a supplementary estimate, signed before the work is done, not after. With staged payments, each stage gets its own act and invoice — which also suits a bank releasing a loan in tranches.
Missing links cost: no estimate means no proof of scope, no contract leaves deadlines and warranty unfixed, no act means acceptance never happens.