The use of a/an is based on phonetics. When you say S-I-P, the first letter will be pronounced as ESS, which is a vowel sound. Thus, it requires 'an'. Larissa Fenand is perfectly correct. And even if s/he wasn't, it wouldn't have taken away much from the sum and substance of the article, which is on the benefits of investing through SIP.
Of course, I do entirely agree with the point that you were perhaps trying to make. Well-written articles are not only a pleasure to read but are also more credible.
