There is an alternative way for you to invest in STI at even cheaper cost than POSB Invest Saver:
DBS/POSB is offering unit trust regular saving plan (RSP) at 0% sales charge for the first $500 per month per fund. However Nikko AM Singapore STI ETF is not under the subscription list, so the closest one to subscribe is Nikko AM MyHome Fund - HomeGrowth, it's 80% in Nikko AM Singapore STI ETF and 20% in ABF Singapore Bond Index Fund.
http://www.dbs.com.sg/Resources/personal/docs/investments/additionalinfo/factsheets/homegrowth.pdf
Comparing two funds annual management fee, 0.5% vs 0.2%, still better than paying 1% sales charge.
How to do it:
Firstly, subscribe an minimal initial amount of $1000, the net sales charge is 1.5%
Secondly, setup the regular saving plan (RSP) to invest up to $500 per month into the fund, it will be 0% sales charge.
Everything can be completed via ibanking including future redemption, there is no redemption charges for DBS/POSB unit trust investments.
Although this promotion is subjectd to renewal once a year, DBS/POSB has been renewing it for the past 4 years as what I can recall.
If you don't like the 20% bond allocation, another alternative is Nikko AMSingapore Dividend Equity Fund, the fund has outperformed its benchmark STI Index consistently and paying monthly dividend at 5 to 7%p.a.
http://www.dbs.com.sg/Resources/personal/docs/investments/additionalinfo/factsheets/nam_shenton_sg_div_eq.pdf