Received a blatant B-list invitation BY PHONE BY PROXY, what to do?

[aephi alum]

Background: My husband and the groom became friends in grade school and have kept in touch. MIL and the groom's mother are BFFs. The groom met a woman and they're getting married in October.

The planned celebrations are ...
- Bachelor party in Vegas, to which my husband was invited. We know the man planning the bachelor party, and he would make "Californication" look like a church picnic. DH turned him down as he is not into hookers and coke.
- Bachelorette party and/or shower - I don't know if these are happening, as I don't know the bride and I would therefore be excluded.
- Engagement party (here's where the shit hits the fan)
- Wedding in October.

The RSVP date for the engagement party was August 5. On August 8, we got a phone call from my MIL, who'd gotten a call from the mother of the groom. We were all suddenly invited to the engagement party and the wedding.

Today (August 9) we received a formal invitation to the wedding. The postmark read August 6.

The groom's family is playing the "your invitation must have gotten lost in the mail" card. Funny how my in-laws' invitation got "lost in the mail" too. The USPS is not THAT incompetent!

What to do? We've already RSVP'd yes to the engagement party and plan on attending the wedding. I am sorely tempted to skimp on the gift, but that would only hurt the groom (I couldn't give two shits about the bride or her family or the groom's family). Perhaps I could ship them something via Am@zon and have it get "lost in the mail"? *evil*