In your sentence ta means 'Has' but ın general we can place -ta/te after the -mak/mek infinitive ending to create what I have always taken to be a kind of alternative present tense form e.g. yapmak .... yapmakta: s/he is doing/making; yapmaktayım: I am doing/making.

"there is a bill dept that is not paid." i dont understand your question.