Was this a proper way of responding to a late RSVP for a kids' party...?

My 4 yr old has been invited to a friends' party down the block. Unfortunately we can't make it due to previous appointments.

We received the invite card about over a week ago and the party is Sunday. I've been meaning to get back to them sooner, but we just had so much going on and people in and out of the house that it kinda spilled my mind.

The problem is, we seemed to have misplaced the invite card, not sure if it was thrown out by accident or what. But anyways its gone.

Because they live down the block from me, I decided to just walk down to their house and let them know in person we can't attend the party.

Of course when I get there, no one is home.

My thought was to write out a letter saying we can't make it and so fourth and also an apology for the late RSVP. They wanted the RSVP earlier this week.

I went back to my house wrote out brief letter and also a happy birthday to the child and let my 4 yr old add some stickers to it, and put in my neighbors mail box.

I feel bad about all this, but we have just too much going on.

What do you think? What was the proper enough? What would you have done?

Thanks in advance :)