I don't know about the coupon code part but if you go to the post from nitelily3 she has a link to her blog with a spreadsheet she used.
I would imagine the same voucher can be reused: IE, a $30 voucher is bought, the first purchase = $25. Then when they come back and use "other" again with the same voucher#, you invoice them for the new Amount, minus the $5 they have left.
I plan to review and or create my own spreadsheet for tracking, for voucher use, and for remaining payment, and payments received (my own "ok to ship" tracking). I've not heard yet about expiry dates on the coupons, but I have heard Hearts provides you with a list of those purchased.