There are two types of setem hasil: setem tertera (aka franking) or setem pelekat which you can buy at post office. Both serve same purpose. Might as well just get the franking one, save the trouble of extra travelling to post office.

It only make sense to buy the setem pelekat if it is for a document that does not require you to go to LHDN to finalize it.

For PTPTN agreement actually you can either get it stamped yourself or can send setem pelekat and they can stamp for you. But that what I know from years ago, probably they do it differently nowadays. To be on the safe side just get it stamped yourself. (safe the trouble getting the document returned to you).

Well just fill the form, get your number and wait to be called. Normally you will get your documents checked at one counter then you will move to payment counter to pay+get stamped. Same procedure whether you use franking or pelekat.

Btw for franking you need to fill form PDS 1, if setem pelekat = form PDS 2. You can actually download these forms and fill before hand.