I had the same encounter too when I called last month. Based on some feedback from this thread, I knew it is based on statement date but due to I was in overseas and had nothing to swipe in April statement cycle, so I got charged with this monthly charges.

Nevertheless, I called and asked for clarification because I did make one transaction on 8th April and my cycle is 12th April to 11th May. So technically, I had one transaction in the month of April but not in the statement cycle. I was expecting answer that my request will be rejected and I will further argue based on T&C. However, the CS put me on hold for a while and after speaking to her superior, she told me it was a glitch in the system and it should be based on calendar month. They had waived it right away... So, I cannot argue further.

So experience from others tells, it is based on statement cycle. Don't trust the CS so much, some said statement while some said calendar month.

Yes, Digi website and even the app can manually enter any amount. I always customize my amount because I'm using HLB Wise to pay. Sometimes below RM 1.

Suggest you to use the app, very easy.