My midwife, a CPM, does the same thing. (I have to pay in full by 36W, then she'll bill insurance.)
You say you "KNOW" it'll be covered at 80%. Why? Is she an in-network provider for Carefirst? If so, you may be able to get from them, in writing, what their max allowed payment is for the global fee ("global" referring to the prenatal care, L&D, & PP care altogether.)
I actually was lucky enough to get an exception! I have a letter stating that they will cover my HB with my CPM at the in-network rate (they max they'd pay for any in-network maternity care provider) because there are no in-network providers available to me who provide HB services.
This obviously is fantastic for me to have lined up in advance, but it can be risky. They can easily say, "We don't cover HB" and then you're left with NO coverage at all!!!!!!!!! Whereas other HB mamas have told me they simply had their MW bill as an "out-of-network" provider without telling the insurance co that it was an HB .... and insurance just never realizes that the, normally separate, "facility bill" simply never arrives.
So it's a bit of a gamble. But it never hurts to call & ask about coverage options and ask, "Do you cover HB?" and if they say yes, see if you can get the exception for in-network coverage rate of Susan if she's not in-network.
Finally, $4K for global fee is high! I thought my CPM was high at $3,700. Unfortunately, even with full-coverage at the in-network rate, I'm still going to have to fork over approx $1,100. That is because insurance companies have max allowed rates. You may have seen on other "explanation of benefits" insurance statements for other services - there is the amount the provider bills, and the amount the insurance company is willing to pay, and the latter is often less!!
BUT... because these providers are "contracted" with the insurance company, they have to accept what insurance is willing to pay. [I think I have all this right - someone jump in if I'm wrong.] Whereas for my CPM, she's not a contracted provider and is free to make ME personally obliged to make up any difference between what insurance pays and what she charges.
Therefore... the "80%" you refer to could be 80% of the max allowed fee for an in-network provider. Which I'm told by my insurance company, based in Virginia, is around $2,600. That sounds right to me because Upper Chesapeake told me they charge $2,700. So if I had 80% coverage, I'd owe my CPM my 20% of $2,600, PLUS the extra $1,100.
I wouldn't let this stop you if you like her & she is convenient to you. If she's usually covered at 80%, then it is a pretty safe bet you'll get that 80% too. Also, with insurance, being denied doesn't mean you are SOL ;) - you submit a claim again! Sometimes if they submit it differently (i.e. not 'global'), they get more coverage. My MW has a billing service who manages the whole mess, so she can keep trying.
Hopefully Susan has someone who does billing for her too. I say just get on the phone with that person & run through all of this, and see what kind of written commitment you can get from insurance about coverage for Susan's services.
Best of luck!