I believe they are a digital first publisher and digital first publishers tend to do everything via email. I wouldn't bother trying to snail mail them anything.
I received the notice of acceptance on 25 May and she requested the hard copy be sent her by 01 June-- and I overnighted it to arrive the Friday before the deadline. Unless a formal electronic signing app is used, it seems they would still need a signed hard copy of the contract for legal reasons and records. A scan attached to an email doesn't always suffice.
And the are doing physical bound copies of the anthology-- the contract said I get 2 bound copies and 5 electronic ones.