Car mechanic did not show me the invoice?

I visited a mechanic today and they told me I need to change one of the engine mounts they called a Honda dealer to ask the price and I accepted to let them fix it.
I searched on the internet and found the genuine part for half the price so I went to the mechanic and asked for the invoice but They did not give me the parts receipt and said that they have the right not to show me the receipt.
I was wondering if they have to show me the invoice or not?
TIA.

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