A guest book fitting for...our guests?!

Ok, lame title.  Forgive me?

It's taken me months to put together the pieces of our guest book, mainly because I refused to spend more money that necessary on each piece.  I've stalked ebay for good deals on postcards that fit our travel theme and spent months thinking about the perfect receptacle for the postcards.  Finally, I have pictures!

I am now the proud owner of 180 blank postcards (and 100 some odd used ones- there are some cool old stamps!) for guests to use to write us a well wish on our wedding day.

There are a variety of black and white and color postcards, as well as ones reppin' the US, Europe, Asia and South America.

I purchased them all off ebay, averaging at about 20 cents per postcard.  I bought about 100 in a large group and the other 150 in smaller groups of 20 and 30.

But what will guests put them in?  I struggled with this too- until I remembered a brilliant idea:

source- the actual mailbox I should have arriving on my doorstep any day now! Only $12.97!

Years ago, Mr. Pencils bought me these cheesy old airline stickers:

I'll cover the white mailbox with the stickers for the travel look as well:

We'll stamp the postcards and have our parents mail them to us while we're on our honeymoon so we have some lovely post waiting for us upon our return.

But I need your help, hive.  Should I have the guests write whatever they want, or should I give them some sentence starters?