Why do I pay monthly?

I have a storefront I just launched and I'm looking for someone to do SEO. For some reason the 3 people I have called so far all say I have to do a monthly plan. What are they doing every month besides the initial time? This one company claims that work on each store 15 hours per month and that's why they charge $1500 every month. I'm a complete novice so I don't want to offend, but it appears that after adding my info to backlinks, sprucing up the website, and adding keywords there isn't much else they do. Any info would be really appreciated.