Is this rude?

My cousin is having a graduation dinner and invited me. I don’t really like my cousin but I feel obligated to go because my aunt has done some really nice things for me lately. Also the party is an hour away and I sorta have other plans that same time that I’d rather attend. Would it be rude to say ‘I’m so sorry I couldn’t make it but please give me your address so I can send a card?’

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