Government agencies are never too keen on telling people when payments to them are entirely optional though. When I was approaching State Pension age, I received a letter showing various gaps over the years in my National Insurance contributions, with an invitation to make up the gaps with a variety of payments (with some of those payments being for under a hundred pounds but with others being for several hundred pounds).
I phoned them up and asked what, if any, benefit I'd get from filling in the gaps in my contributions. (i.e. I wanted to know if I could increase the pension I'd receive). I was told that, as my State Pension was already at the maximum anyway, there was no point whatsoever in filling in those gaps (which the letter had failed to make clear).