Japheth, it really is as simple as the guys above say--but me personally, I like to include a list of all my prior up-pledges along with it. For example, here's the one I sent last night for decks:

Just added another $60, two more Terrain packs and an advance on four extra Capt/Crew Decks.

Total current pledge and addons so far:
$260 - Captain package
$40 - HMS Victory + USS Constitution combo
$20 - Extra Mat
$5 - Extra Attitude Indicator
$5 - Extra Combat Ruler
$15 - 3x Extra Terrain Pack #1
$20 - 4x Extra Captain/Crew Deck

Thank you,
[NAME CENSORED]