The right support package depends on the length of time your site has been live and the revenue you're generating per year.
If you're a start-up or new store and generating less than $500K p/year then the PAYG package (hourly rate) is best.
This is so you can allocate more of your budget to building an audience and less to maintaining the website.
If you're generating more than $500K p/year, you should consider a monthly plan so that you have priority support and additional budget can be allocated to site enhancements that will improve your key metrics.