Don't be a fucking retard. If you don't want to send a gift don't send a gift. Did you ever think that maybe they sent you the invitation because even though they knew you would not be around, they didn't want you to feel left out.

OR, they forgot you were going to be away.

Or they don't care.

Seriously, at this point, I'm pretty sure they are not thinking about you and don't give a shit.

Why are you just getting the invitation now if the wedding is in August???