Would I be offending the couple if I wasn't invited but sent in a card?

My best friends sister is getting married soon and my family wants to send in a card congratulating her and her soon to be husband. We were not invited, they are purposefully keeping it family only because they are doing a smaller wedding and they already have 30ish family members.

Our families are very close, we have get together events 1/2 times a month, have thanksgiving, Christmas, and 4th of July together, and I have known them since the 2nd grade (I am graduating college soon). I would drop it off the day before or morning of and leave, so no surprise appearance. I am skeptical because they intentionally kept it a private and close wedding.