ups sent me an customs invoice via mail after month after delivery after i’d already paid customs prior to delivery..

so i imported a couple big boxes internationally via ups express almost over a month ago. i paid the customs for all of the packages before they arrived, which i’m aware that they make you pay before delivery, so i assumed i was fine with the fees since they all delivered with no issues. however i just received an invoice via mail for customs for one of the packages (which was supposed to be paid by sept 14 but i only just received it), and i’m assuming an invoice by mail will also be sent for the other packages.. is there a reason they do this?? and should i pay it?? idk what to do because 1) it’s overdue already and i can’t do anything about that because it only JUST delivered and 2) why am i paying customs again a month after delivery when i already paid for “everything” before delivery..