If I understood well, 月末締／翌月末払 means "close the deal before end of a given month then get payment before end of the following month"
Is the pronunciation "げつまつしめ よくげつはらい"
I am especially unsure about 締 and the い for 払.
I would read it as げつまつじめ、よくげつまつばらい. I was surprised to see that Chocolate reads 月末締 as げつまつしめ, but I suppose that it is either personal or dialectal difference.
Okurigana of a noun such as め in 締め and い in 払い is sometimes omitted, especially when the word is used as part of a compound word.