Do you have online access to your mortgage information, so that you can see when the payment posts to your mortgage? Or at least a phone number to call to find out balances, last payment, info like that? I've made a test payment of $100 to my mortgage company in the past with Serve, and it ended up being applied toward principal. I was fine with that since I was just testing their system to make sure they received payment, so I didn't have any issues. But you should check with your mortgage company and when they receive the funds, make sure that they are applying the funds toward your mortgage payment and not just your principal. I've been using Plastiq for mortgage recently, but even with sending mortgage payments through Plastiq, I've sometimes seen it applied toward principal only and I've had to call to have it reversed and reapplied toward the mortgage payment. I use Serve and Bluebird all the time to pay cc bills.
I have a Bluebird, and manage DH's Serve account, and since I only use WM anyway the two services are the same as far as I'm concerned.
@AmyAnne ETA: Except for the fee!! I make sure to drain the green Serve account down to zero before the fee hits. Bluebird doesn’t have a fee.