3 ways:
1) maintain autobilling ----- on weekends
before bill date, pay outstanding [minus] rm20+, leave balance just about rm20+. this way, when autobill run, it will pay balance rm20+ and grant you rm2 autobill rebate for next bill.
how to know your current outstanding before bill date:
a) if many usages (e.g. call charges or VAS), can check in Unifi OCC ~1 week prior to bill date. you will see the current outstanding, then roughly you'd know the amount to pay in advance. why 1 week? at least you got yourself a spare weekend to pay the unifi bill!

always check the calendar...
below is a simulation i did for my Unifi. an average of 3-4 days whereby the previous weekend falls before bill date on the 7th

the formula is tweaked so that i always pay on previous Sun, if possible. if bill date falls on Sun, then the Saturday before.

b) if you hardly use anything else with unifi, you sure know the fixed bill amount every month.
2) maintain autobilling ----- do nothing. hit-or-miss for weekend transaction. easy to maintain but no gain!
3) cancel autobilling ----- manually pay all outstanding on weekend. you have about 2-3 weekends before payment due date (21 days after bill date).
Mine not yet billing date, I simply paid first minus out RM20 + RM2 rebate.
Since you've been paying this, can I just clarify what is the amount I'd pay using my AMEX for a total bill of RM106 inclusive GST?
I just paid RM84 (RM106 inclusive GST - RM20 min payment - RM2 rebate). Is this correct? Just ain't sure if I'd pay RM86 or RM84 using AMEX - to include or exclude RM2 rebate.