The question you specifically ask relates to fairness, which calls for a rather subjective response. In my view it would be fair for you to be included among the authors, but I also know that some research teams would be more inclined to list your name in the acknowledgements than to put your name in lights along with theirs! On the issue of fairness, I think that the best you can do is argue your case with the other authors.
There are, however, two other issues that are distinct both from each other, and from the issue of fairness. The first is the matter of so-called moral rights; the second relates to the policies of different journal publishers. In those jurisdictions that recognize moral rights, perhaps the foremost right is the to be attributed as the author of a work. For example, in Australia, moral rights (generally) are granted under Part IX of the Copyright Act 1968, and the right of attribution is granted under s. 193. However, it is not all as simple as might first appear. In your particular case, for example, your moral right to be attributed as the author of the software would not arise merely as a consequence of the software being discussed in the paper. Instead, your moral right of attribution would come into play only if the software itself were being published or distributed. In that case, under Australian law, you would be entitled to require that you be named as the author of the software, even though you had been paid for your work, and even though you no longer owned the copyright to the software.
In addition to the matter of moral rights, there is also the question of the policies of different journals. Most publishers require that anyone who makes a substantial contribution to a paper be included in the list of authors but yet again, it is never as simple as first appears. Some journals will view the development of an important piece of software as fulfilling the criteria for a substantial contribution, others will not.
It is worth mentioning that statistical consultants often find themselves in a similar position to yours. Much of the work that statisticians do is viewed by some researchers as "mere technical assistance"; other researchers will recognize it as having been essential to turn data into useful conclusions.