I have the receipt.
'Scontrino' — stress the second syllable: scon-TRI-no. The 'sc' sounds like 'sk'.
Say this immediately when asked for proof of purchase. Presenting the receipt proactively speeds up the process and signals you are a straightforward customer.
The definite article 'lo' is used before 'scontrino' because the word begins with 'sc' (an impure s-cluster). 'Ho' is the first-person singular of 'avere'. The receipt is called 'scontrino fiscale' in formal contexts — the fiscal receipt required by Italian law.
Ecco lo scontrino.
Here is the receipt.
More direct — hand it over as you say it.
Ho la ricevuta.
I have the receipt.
'Ricevuta' is used for digital or bank receipts.
Ce l'ho qui.
I have it here.
Informal, when fishing in your bag.
In Italy the scontrino fiscale is a legal document. The Guardia di Finanza (financial police) can stop you outside a shop and fine you if you cannot produce one, so shops are strict about issuing them.