Stripe invoices

I have a lady that orders items from me that I don’t have listed on folksy. She contacts through e - mail and she doesn’t have or want a paypal account but has paid on her card through paypal.
I have added a seperate account through Stripe and set up invoices but I am a bit confused as they keep mentioning an api key? I sent a test invoice to myself which has everything there but being a test one you can’t know for sure if it works. Don’t want to tell her about it and look foolish if I haven’t set it up right.
Has anyone else sent invoices using Stripe?