Today, at a clothing store, I approached an HB8 while she was working. Because she was on the job I knew I had to make it quick to avoid interruptions. She wasn't wearing a her tag so I teased her a bit for being a slacker. In short, she gave me her email and phone number in about three minutes, but strongly preferred that I email her (because her brother spilled lemonade on her phone and it "messes up" w/e).
She was totally into me but I've never done the 'email thing' before, so what should the first email to her be?

(I could easily call her, leave a message if she doesn't answer, but I'm looking to expand)

It's a tricky situation. You only interacted for 3 minutes, so you can't seem too interested in her. Otherwise she wonders why you would be so interested after only 3 minutes.

I'd much prefer a text, because that can appear spontaneous, e-mail is much more intentional.

I think your best bet might be to "invite her along" on some plans that still exist without her. Like you're having friends over for dinner/a party whatever or are planning on going out with a group. That way you can invite her to join and it feels so much more casual than a date. Maybe even suggest she bring a friend to further take the emphasis off her.