I'm going from memory here:
Total Purchase Requirements:
Level 1: $100.00
Level 2: 500.00
Level 3: 750.00
Level 4: 1000.00
Level 4 is the highest level. To see what you can get at different levels, you have to have ordered at least once before. The current shopping cart amount counts towards your current total, so if you've ordered once before, just fill your cart up with enough stuff to hit the mark.
For example, if you've ordered twice before and your total of both orders is $300.00, put $200.00 worth of stuff in your cart to hit $500 total, go to checkout, and it'll show you what you can get at Level 2 without having to submit the order.
I put $35,000 worth of stuff in my cart once and it still didn't go to level 5 so I think I'm stuck at 4 for good. The nice thing about the higher levels is that the stuff you get can exceed shipping costs. Total shipping is about 17 bucks these days, and at level 4 you get items worth $20-$30, so it covers your shipping, as long as it's something you want.