You just have to be firm. If buyer has written and refused to upgrade shipping...
just nicely something like, I see you have decided not to upgrade shipping. From there just let her know you can mail it regular and the the estimated arrival date, stressing you cannot guarantee delivery time.
I would offer to refund if she doesn't want to upgrade shipping and does not want regular mail.
If buyer has not outright stated they will not upgrade, I would send them a PayPal invoice for the upgrade and follow up with-- if payment is received for upgrade by X date, then item will ship Y date with expedited shipping. If this does not suit your needs let me know and I can either ship regular mail or cancel the order and issue a refund.
Just clearly let the buyer know what the options are.
Personally, I would not ship until I received payment for expedited shipping or confirmation from buyer regular mail is okay. Be sure to let the buyer know that too.
Good luck with it.