What coupon to use
 

Say I were to buy 3 dvd's for $35 each, total $105.
1. Should I use the $10 off $100 coupon, probably not.
2. or buy each dvd in 3 separate orders and use the $5 off $35 coupon
3. Or can I just input the $5 of $35 coupon 3 times for 1 order?

Definitely go for route #1.

1. $10 off the entire single order, plus free shipping.

2. 3 separate orders: 3 x ($5 off - $2.98 S&H per order) = $6.06 off all 3 DVDs.

3. Not possible. Amazon limits 1 coupon per order.

They also only allow a coupon to be used once per account.

#1 is definately the way to go, since you'd qualify for the supersaver shipping and with #2 you'd have to pay shipping for each DVD...

If I use a promotional certificate on an order and decide to cancel that order later on, would I still be able to use the same promo certificate (if it hasn't expired) on another order?

I have found that when I cancel and order with a promo certificate, the $5 or $10 discount coupon always appears on my checkout page next time I place an order.