Does anyone tried redirecting custom button in suitelet to 'POST' instead of 'GET'? Any reference would help.

You can add a submit button, that sends the post request to suitelet.

yeah I already have a submitbutton.. Can I use a mutiple submit button in Suitelet?

You should be able to add multiples, but not sure if they can perform different operations.

Add a custom button, add client script for that button function, client script function performs a https.post

you can use HTMLElement.click after getting the submit button element using one of the element selectors like Document.getElementById