Learn English – word for the day before the last day of a period, which is measured by days

Another option is to refer to the payment directly and to the number of days indirectly allowing you to use the term:

in arrears: the state of being behind in the discharge of obligations < in arrears with the rent >

You may then say things like:

My payment is two days in arrears.

In a tabletop setting, we would call that kind of player "powergamer" (when it's more skillfully using the rules to maximise some effect) or "munchkin" (when it's hurting fun for everyone else).

So I suppose those terms might work; say "don't camp, it's powergaming".

A less games-focused term might simply be "cheap".