Pos Ekspress will be the cheapest. Just the RM3 envelop will do. Make sure no packaging.
As Pos Ekspress is now run by Poslaju, they have been very strict with the envelop limits (i.e. size and weight).

Yes true, but Poslaju will cost RM5.95 to WM.
Also, do note that Pos Ekspress do not need signature upon delivery. Some of your customers may still want to use Poslaju.

Which courier method your seller used? And where did your seller post it out?
Big post office usually close around 5.30pm, so no way he could've post registered post. If it's by normal post and he drops it into the letter box at 7pm, then it will be processed the next day.
If by Poslaju, Poslaju offices usually close around 8pm, some closes at 6pm, normal post office cut off time is at 4pm. So if he post at Poslaju office at 7pm, then chances are you will get it the next working day.
If by Pos Ekspress, if he drop it into the yellow box at 7pm, it'll be processed the next working day. If he pass it to the Poslaju officer at Poslaju office, then chances are it'll be process on the same day. Pos Ekspress has different deliver time to different zone, it's next day delivery for within Klang Valley.
If by Parcel Post, then it'll take roughly 3-5 working days.

If by other courier companies, then hard to say. It depends on their own courier company how efficient they are.

If no tracking number, then just hope that it'll arrive one day. Nothing much you can do.

I know normal mail from UK to Malaysia takes around 1 month, sometimes can be up to 1.5-2 months.