This all felt a bit much for a time of year when the most stressful thing I normally face is 'working out if I can fit in a fifth mince pie', or whether it's best to leave it at four.
I'm confused with this sentence because of the words 'work out' and 'fit in'. The author got a notice letter of 'violation fine' from Transport Service and she's very frustrated and upset and especially because it was Christmas Season. I can feel that she wanted to express her feeling in humorous way, but I'm not sure about details.
Does she mean that working out (figuring out?) if she can fit in (go well with) fifth pie or just have four pies? is it like expression that couple break up or something?