Can I have a negative balance due to the monthly fee?

Definitely no!

If we couldn’t charge the monthly fee for your active card/cards due to the lacking funds, we will deduct the fee for all the months when it hasn’t been charged once you have sufficient balance on your card.

Thus, if, for example, you have purchased a card on the 1st of January, but credited some funds on it on 5th of May, we will charge you 5*4 months (for January, February, March, and April).