How can i get through to the lady that i provide child care for?

i'm about done with the lady that i provide child care for.every payment option that there as been,as been in her favor by far.i couldn't do 50 bucks every week and so i told her it had to be 300.i've gone from only charging a dollar an hour to maybe 1.50 to finally saying it's gonna be a flat rate of $150 period.every two weeks that's what she pays me and still thinks she pays too much.she still owes me for the first month that i started watching her kids and now she says she is only gonna pay me half of that.why?well,she feels that since she went on vacation the week before and i only had the kids for two days this,then that's all she should pay.the way i see it,vacation or not,flat rate is a flat rate.watching her kids is my job and i get paid,so that's income.she gets paid to go on vacation then so should i.her days off are my days off.that might not apply in a normal situation but the whole thing is far from being normal.she doesn't even pay half of what the normal cost is for daycare.if anything,she could be paying me for her past dues.i don't feel that i'm in the wrong given all the crap that i've had to deal with.i just need some advice on the matter before i tell her that she needs to find another babysitter.