I want to subscribe via a membership card but I also plan to resubscribe at the beginning of next month. I want to know how payment works in that if can have payments done on a certain day while I have membership set up already.
If you have a recurring subscription going, Jagex take the next payment
three days
before the end of the subscription.
So, if you want to always pay on the 1st of each month, you would have to start the first subscription on the 4th of the first month you pay. This means the following month the renewal is taken on the 1st.
If you pay now through a membership card, you would ruin what you are trying to achieve.
Example: You pay today with a Runescape PrePaid card (lower value card) and get 25 days of membership. Even if you pay and set up a subscription on 4th January, the system would count the days you already have paid for with the card and would not charge you again until 10th February.