I can try to answer your questions based on info I have gathered here on the boards (going for my first trip in 2 months!)
1. I believe that the advantage to ordering from staples before was that they offered free shipping for members, however you now must purchase at least $50 worth of stuff to get free shipping. Some people get around this by ordering something small and spendy that they need or will need anyway, like ink, memory cards/sticks etc. If all you need is the water, I would check the delivery fee for both staples and garden grocer and then make your decision. Personally, we are probably going to order from garden grocer as we have heard good things about their level of service, and we can also get a few breakfast items for in the room!
2. I think the advantage of doing the cd at home is you can really take your time to add borders, effects and crop you r pics at home where as you might feel rushed at a park (not to mention wasting valuable ride time!) I think from what I have read that the features available are the same whther in the park or at home. I believe the cost for the photopass cd is $125- and this is for all your pics, modified as well as original. If you need more than one cd to hold all the pics it is included under the $125, so no additional charge.
3. Most Disers who buy from ebay are looking for lots of pins (I have seen 25 or 50 pins most frequently) where the cost per pin is about $2. You couls probably get a smaller lot- like of 10 - for about $2.50 per pin. You can also buy pins individually but they will probably cost a little more. You are looking for genuine Disney pins, not Sedsma and the like. Also, I have heard it recommended that you look for lots that advertise not a lot of repeat pins, as a CM cannot trade for a pin they already have on their lanyards so you will want a variety. If you look on the pin trading board- I think it is near the bottom of the forum list? - you could get some good direction and more detail, as well as a list of reputable sellers from ebay. Or hopefully, someone else can post here.
Good luck with everything!
