She wants to put off my full payment -- should I say yes just this one time?

[5 Little Monkeys]

Again, just my opinion, but if you are going to stew over this and resent her for it than don't do her the favour. If she's gone through a couple other dc's already there must be a reason why. If you happen to know who they are maybe you could contact them and ask?

I understand we all have different priorities but IMO bills come before birthdays, especially at such a young age where they will not even remember it. It also wouldn't be the end of the world if she paid you on time and celebrated the birthday a different day.