This is not about service levels in the sense of a service level agreement (SLA), although there is a lot in common. In a translation context a service level agreement would define how many errors of what kind are more or less inevitable and should be accepted as a fact of life (yes, nobody's perfect and you won't avoid every single type over 300 pages of perfect writing without as much as a speck of bland style or boring narration, let's get real), at what point the service would still be good but require some adjustments, when it would be substantially good but justify a discount here or there in addition to remedial work, and finally a more elaborate and less passionate structure of contractual penalties. If you take a standard contract and outsource the part about contractual penalties and give it more flesh and narrow down some technical aspects, delivery and so on, that will be an SLA, essentially.
This may seem to confuse 'service' in the understanding it typically has in everyday language (what do you think about when you hear that something was good or bad service) with the more narrowly understood product of what really is work for hire.
And translation does differ from 'services' in this, it is a work for hire. In a legal sense, 'services' are also often work for hire or commissioned work but real life focuses on non-legal aspects more. We aren't a branch of the hospitality industry (hotels, bars), nor a concierge service (where you literally get paid for making the client satisfied because his wishes of various sorts are met).
Client service perhaps still exists in our job line, but it's something markedly different from the core, substantive quality of our work. Does anybody talk about client service to architects? Lawyers have client care for obvious reasons — in some situations their clients are essentially patients. Patients invoke associations with hospitality. This applies to interpreters to a moderate extent, but that's it.
Whether it's service with a smile or with a scowl, it doesn't even matter and shouldn't even have popped up — the way clients are treated reflects on the individual translator's character, as in personality, manners and so on, but the whole thing is about translation, not about creating experience or providing care, forget simply running errands, making requests happens and procuring general satisfaction.
Bottom line: 'Client service' is quite incidental to our profession, generally the province of receptionists and secretaries that most of us don't hire but agencies and clients do, and is not to be confused with the substantive quality of our work, which is always focused on rendition from one language to the other.
You can't really produce unfaithful translation or bad writing on purpose, so in this way you can't 'sell' different quality levels or plans. What you can do, however, is itemize the price instead of making it all-inclusive or, in other words, inclusive of certain requests that your client might or might not make. There is no imperative (ethical or otherwise) that you have an all-inclusive price. There may be an imperative that you are available for this and that, but never that you are available within a low price or within the price already paid.
Fixing errors in your own work would be hard to charge for — although there are situations in which even that could be appropriate (e.g. rush jobs, stylistic issues in working translations) — but you can definitely charge separately for:
reviewing mere proposals of change, as in whatever changes your client thinks about or even the proofreader or editor, as long as they don't come down to clear-cut errors on your part;
your time spent dealing with corrections that shouldn't have been suggested, i.e. explaining why the proofreader is wrong, i.e. free grammar lessons and that ilk;
explaining your choices when more information is sought — you should be able to explain them, but not necessarily free of charge and from a position akin to that of a criminal suspect;
and probably some more.
Right now agencies and in some cases also clients are pushing for lower prices and shorter deadlines. So perhaps give them that, but the lower price doesn't necessarily need to mean the same kind of work or client service that would normally have been included in the old, higher prices. You shouldn't have ethical objections here: it's not somehow fair for them to impose a lower price on you but still claim the same old thing — it works both ways.
If they don't want to pay more just in order that some things can be free (i.e. already included) when they pop up — fine, that's a valid choice. But charge them when those things do pop up. You can charge them high, as in for separate service, or you can tell them to make up the difference (upgrade their service plan, as it were), but you don't need to agree to do the same work or more for less. Especially if that work is not your core work, i.e. translation, but some transactional, administrative or educational aspects — especially something someone else could handle.
Notably, queries and requests from end clients that mostly come down to explaining the rules of the language to them can be handled by the in-house employees of any self-respecting translation company, at least as long as the most popular languages are concerned or otherwise languages their inhouse translators and proofreaders can handle.
There's no need for you to handle secretarial or assistance kind of taks or fill technical billets. You're a translator, not a software techie or office secretary, those are just billets the agency isn't filling in order to save money and cut a larger margin or be able to lower its price for the clients and thus gain a higher market share. Which is not your responsibility.
Admin is their job, not yourself, and so much more so concerns such as keeping the client happy, especially by humouring the client and granting all sorts of less than reasonable respects — down to what, entertaining the client and making coffee? Those are all valid roles and valid business decisions, but the people who deal with the execution of that are either specialized hospitality employees (so to say), who might even be making higher pay than substantive staff (a good, experienced receptionist is probably worth more than a generic new lawyer in a law firm, for example), or perhaps junior office staff. You may be junior office staff if you're in your early twenties with fresh ink on your bachelor of arts (yes, lowercase) degree, but not when you've got an advanced degree an a serious job in the professions.
For the record, those admin, technical and hospitality roles are not even something you need to provide for agencies or to their clients on their behest at all. Let's get some things straight already.
Bottom line: just like when a direct client doesn't like the prospective bill: itemize. Making it give and take. If they give less, they aren't entitled to take all. Draw your limits.
And you may need to put those limits in writing, as well as in your ToS, available somewhere handy on-line. This is because the law is slow to adapt and operates on a series of presumptions that are no longer valid in the situations we meet these days. Keep things clear so presumptions can't be made.