Should there be a comma in 'a 30-day, money-back guarantee'?
Solution 1:
A comma is usually used between adjectives that are parallel, that is, modify the same head in a noun phrase. This usage is essentially just a listing comma: you put commas before all entries in a list (including or excepting the final one that also has an and, depending on whether you like the Oxford comma or not).
Example:
A small, red, wooden(,) and cozy house.
Here, we have four adjectives that all modify house: the house is small, it is red, it is wooden, and it is cozy.
If we make a copular sentence out of this phrase, it becomes:
The house is small, red, wooden, and cozy.
Conversely, no comma is used between an adjective and the thing that it modifies, so there is never a comma in “a wooden house”. It is important to note that adjectives do not just modify nouns: they modify noun phrases. And a noun phrase modified by an adjective is also a noun phrase. The following, then, is also perfectly correct:
A small red wooden cozy house.
This means something different, though: it means that there is a [cozy house] (a noun phrase, a single unit), which is wooden. This [wooden [cozy house]] (another single unit) is red. And the [red [wooden [cozy house]]] (single unit again) is small.
If we make a copular sentence out of this, it becomes, in ‘stacked layers’:
The house is cozy.
The cozy house is wooden.
The wooden cozy house is red.
The red wooden cozy house is small.
In this example, that gets a bit fanciful. It’s more natural to just describe a member of the category houses as being all four things, rather than take a member of the somewhat fanciful category red wooden cozy houses and describing it as red, etc.
In your example, however, things are different:
A 30-day, money-back guarantee.
A 90-day, no-risk trial.
With the comma, these phrases describe a member of the category guarantees, which is then said to be both 30-day and money-back; and a member of the category trials, which is said to be both 90-day and no-risk. Without the commas, the categories are money-back guarantees and no-risk trials.
In copular form, with the comma:
The guarantee is 30-day and money-back.
The trial is 90-day and no-risk.
And without the comma:
The money-back guarantee is 30-day.
The no-risk trial is 90-day.
Of these, the latter make more logical sense to me, but there’s nothing actually wrong with the former. Either version, with or without the comma, is perfectly correct; they just mean slightly different things.
(Neither 30-day/90-day, money-back, nor no-risk is really a true adjective; they’re more like noun adjuncts. This means that they are limited to being used attributively, rather than predicatively, so the copular sentences here are all quite strange-sounding, no matter how you phrase them.)
Solution 2:
A general rule is that a comma should be used if the two modifiers both modify the noun, rather than the first one modifying the noun-phrase formed by the second modifier and the noun.
A heavy, bulky box.
A lovely hand-made toy.
Heavy applies to box about as much as bulky does.
Lovely applies to "hand-made toy".
Three guides can help decide if you have the first ("coördinate modifier") case rather than the second ("cumulative modifier") case:
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Could you put and between them without changing the meaning (but being wordier and hence changing emphasis).
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Could you switch them around without changing the meaning.
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Are they at the same level in terms of modifier order (amount or number, general opinion, specific opinion or quality, size, age, shape, colour, origin or material, purpose or qualifier, noun-as-modifier).
These essentially amount to the same thing; they're different ways of spotting the same thing going on.
(They can be complicated if a pair of adjectives become idiomatic, as then re-arranging will sound wrong even though its grammatically fine).
The question of modifier order is complicated here because nouns turned to modifiers ("30 days" to 30-day, "money back" [from the noun money] to money-back, "no risk" to no-risk) always tend to come at the end, and since we've pairs of such we're likely to order between them as we would adjectives.
I can't see "money-back, 30-day guarantee" being used, but I'm honestly not sure if that's because 30-day is a size and money-back a qualifier or because "money-back guarantee" is an idiom I'm used to hearing. I'm slightly more inclined to have "no-risk, 90-day trial", but not a lot and I wouldn't expect "90-day and no-risk trial".
In all, I think "the money-back guarantee is for 30 days" is more plausible than "the 30-day guarantee is money-back"; the modifiers aren't at the same level, they aren't easily switched, and they don't work equally well with and.
So:
30-day money-back guarantee
90-day no-risk trial
Are the "correct" versions.
But the scare-quotes around "correct" are there for a reason.
If I'm trying to sell something and to use the guarantee or trial to do so, I may very well want to emphasise "30 day" and "money-back" as individual qualities. Likewise "no-risk" and "90-day". A pause between the two will help do that, and that's a perfectly valid reason to use a comma:
30-day, money-back guarantee
90-day, no-risk trial
Screw the rules, I've got rhetoric!
(It's also a reason why I might choose to put "no-risk" in front and have "no-risk, 90-day trial", as much as putting a length first would be more common, as it can allow for more emphasis on "no-risk").
Indeed, I think this is in itself a good example of the difficulties of talking about rules with comma placement, as we've a perfectly good rule that matches most usage that would have us not use a comma but also a perfectly good reason to put a comma in anyway.
Solution 3:
I would use a comma. Two things define the guarantee - that it lasts 30 days and that money-back is the compensation.