I would expect to hear this phrased as "if you need to renew the service, you will have to buy a $100 top up".
I don't think that "top up" as a verb meaning to add to your (usually mobile phone) account is old enough to have a non-idiomatic usage guide, at least not one that the public would be expected to know. Nor, of course, would my use of "top up" as an object noun.
Writing style guides may have created a preferred style.
Companies (especially companies in the mobile phone space, who sell these top ups) definitely will have created their preferred style, and will expect their employees to use it.
But like anything else (see "xerox as a verb"), what comes down from on high may or may not be what develops from the street. Especially in English.