Almost nothing stacks anymore. Used to stack a ton, but now they keep closing another loophole every few months.
I had $20 off coupons that used to work on 50% off sales. Not anymore. If you use one of those, it puts everything back to full price.
I used to be able to add free items onto an order that met the free shipping threshold and get free shipping on the free item also. Not anymore. If you add a free item, even with an order of $39 or $49 with free shipping, it adds shipping for the free item.
You can save a bit by looking for discounted Shutterfly gift cards. I like to use my Discover points to get $25 gift card for $20. Or, as a prior poster said, wait for the free extra pages. They only started doing that a year or two ago, but now it's my favorite promo. Just have to make sure you don't wait until really close to big holidays when they want you to pay for all those pages.