Don't worry, there is no more advance purchase discount when you purchase directly from WDW so you can purchase tickets at POFQ without loosing a discount. This may work better for a large party because each family may want a different number of days and options.
There are some good reputable online companies that will give a small discount and free shipping, I have used
undercover tourist three times and would recommend them. If you don't have time for shipping you can pick them up at a couple locations in FL just google the name if your interested or check mousesavers.com.
I would do some research with regard to what you want to purchase in advance as there are soooo many options. Allears has a great resource page on tickets inluding all the options and prices;
http://allearsnet.com/pl/ticket.htm#2
I would recommend purchasing the tickets at the resort rather than guest relations outside your first park. You will be inside and the lines won't be as long so you can take your time and make sure you purchase what you need. Remember that you can add options to your tickets anytime within 14 days but you can't downgrade.
HTH
TJ