You get a discount for the more months you buy at a time, basically. It's listed part way down the upgrades page:
fantasyfeeder.com/upgradesspecial discounts: 3 months save 30%, 6 months save 40%, 12 months save 50%
The $5 is marked with a superscript 1 next to it, showing one should check at the bottom of the page for this:
1 Based on a 12 month purchase. Prices in US Dollars.
