The invoice they ignore
Work delivered and accepted, then silence. A claim for breach of contract, the cleanest case in small claims.
Paid in part, then nothing
A deposit or first installment came through. The balance never did. Partial payment proves the deal; it doesn’t erase the debt.
No contract, just the work
A handshake engagement, done in good faith. Quantum meruit lets you recover the reasonable value of what you performed.

