The leaping boar statue treasure item comes free with every pledge at the $21 level on up, and is available as an add-on for $2 if you want to buy more.
The shields come free with every pledge at the $45 level on up, and if you want more shields they are available for $1.50 each.
The Merchant's pet lizard comes free with every pledge at the $21 level on up - he is NOT available as an add-on.
If you pledge at a lower level and buy add-ons that get you above $21 or $45 total (for the lizard, leaping boar, or shields, respectively), then you still get the free items that you qualify for.
Any pledge at any level qualifies you to buy the leaping boar statue or the shields as add-ons.
The pet lizard is not available as an add-on, you can only get one of him, and only if the total of your pledge + add-ons is over $21.