Catalogue credit charges

[Chickadee]

I have a catalogue account with a fairly small balance owed on it.

I make a payment each month when the statement comes.

However, I have noticed that the higher my balance the less they seem to charge for interest (they call it service charge).

As an example: for two months my payment was the same amount �25.55 but the service charges were �7.55 one month and �10.40 the next.

Can anyone explain this to me please?

Thanks.

[factor-fiction]

Have you asked the catalogue company?