Declining A Bar Mitzvah Invitation

My co-worker just emailed me and my boyfriend an invitation to her son`s Bar Mitzvah. I am completely surprised to receive this (1) because all she has done at work every day for the last eight months is complain about how much this is costing them since they have to invite the entire congregation as her husband is a cantor at the synagogue and (2) to be completely honest, I don`t consider her a friend at all.
I just mentioned to her that I will be out of town that weekend - but do I need to send a gift? I honestly think the only reason she invited me is that she knew I wouldn`t go & wants the $$ for her son. Help!