Overpaying mortgage

[yodermon]

Well since my LTV is too high to refi (not underwater though -- yet), I'm thinking of overpaying my mortgage to maybe pay it off early. I want to make sure there's no overpayment penalty, and that the overage actually goes towards the Principal, so I sent an email and got this reply:

After reviewing your loan information, I have found that there is not a
prepayment penalty on your loan.

When a payment is received without instructions for application, the
payment will be applied in the following order:

Principal and Interest
Escrow
Late Charges and Other Fees
Non Sufficient Funds (NSF) Fees
Corporate Advance
Additional Principal Curtailments

Ok so my naive question is well, is it Principal or Interest that will be hit first with an overpayment? Or will an overpayment somehow be split between the 2 in proportion somehow? And what's with the "additional principal curtailments" at the very bottom, isn't that really what I'm aiming for ? It almost sounds as if they'll fill up my Escrow account before applying towards principal. Or if I attach an "instruction for application" to the payment saying "apply it to P" then that'll suffice...?

Obviously I should talk to them directly but I thought I'd throw it to the great DU collective before sitting on hold forever and arguing with a rep.
This is a Wells Fargo (gag) mortgage, BTW

thanks all!