It's not what you said it's how you say it!
I'm sure we have all heard that many times!
Try getting her on your side, try saying something like this...
I appreciate that money can be tight in this current climate, interest rates going higher and higher...If you don't mind me asking is that one of the reasons you are choosing the two monthly option?
The two monthly price would be £34 thats an increase of only 25% (ish!)
I appreciate that's a little more, but look on the bright side...
Your actually saving £20 over that two month period! :
If she say's no ...no harm done! but let her go, even offer numbers of other window cleaners in the area.
This way you leave the door open for her to come back!