If you create a Professional selling plan account, a monthly subscription of S$29.95 (w/o GST) is part of the standard Amazon Services Business Solutions Agreement that you agree to. Please refer to the terms of your agreement for more information.
You will find more information about fees in the Selling on Amazon Fee Schedule.
If you would like to sell on Amazon without the monthly subscription fee, you can switch your selling plan to Individual and instead pay a fixed per-item fee when the item sells.