Is there a term for something less than "high availability"?

Increased availability.

Always on?
No down time?
Massively redundant?

Here’s a service level agreement where the provider will compensate clients for downtime. Maybe you could give them something like that, with a top tier that is not as high as 99.9%

Web Site Availability Credit Percentage
99.9% to 100% 0%
98% to 99.8% 10%
95% to 97.9% 25%
90% to 94.9% 50%
89.9% or below 100%

Scroll down half-way to ‘Remedy’
http://www.teamits.com/internet/webhosting/policies/uptime.php

“Adequate availability”, perhaps?

I think people usually handle this by making it sound “premium” instead. “Preferred”, “prioritized”, “exclusive”, “improved”, etc.

Our cable company charges a lot more for any business class internet, when asked the difference they tell you business traffic gets “priority” on the network and then define their customer service commitment.

You might still need to define the terms somewhere, but if you want something clean for the marketing copy, I think that’s usually how it’s solved.

It would depend a bit on what you’re offering, but you’ve got that figured out.

“Duty cycle”. That’s another industrial phrase that indicates the level of utility that a machine or product may have to function at to be acceptable.

In electric power generation a “base-load” unit is one that is expected to be “on” (with a 100% duty cycle) at all times except for routine maintenance, refueling (in the case of nuclear power plants) or other scheduled down time. A “peaking unit” is one that only comes online at times of peak demand, when it’s “all hands on deck”, so to speak.

Pretty good availability?
I am reminded of Garrison Keilor’s description of Ralph’s Pretty Good Grocery Store in Lake Wobegon. “if you can’t find it at Ralph’s, you can probably get along without it”