Developmental Editing - has anyone used "Tandem Services" (Jennifer Crosswhite)?

I submitted my manuscript with them over 9 weeks ago, paid $1,750 of the amount due ($3K), and now they are asking for the final payment ($1,250) without a thing in return.
I am fearing I have been taken for a ride…

Has anyone heard of or used Tandem?
Is this normal business for a developmental edit (my first)?

Of course, I do not plan to pay the final $1,250 without seeing my edited manuscript.

Mike