Let’s start with some data:

 

The Guest Checkout should be the prominent option (see best practice #210).

 

One way to mitigate checkout abandonment and not being able to message them is to ask them for their email address as the first step of the guest checkout.

The same email address should work with multiple guest checkouts, to allow the same person to make more than one guest purchase with their email address.

 

Offer optional account creation after checkout, in the Thank You page. (see best practice #131)

 

Update April 26th, 2020:

Inflow has seen that streamlining the checkout process by removing the account / guest step gets positive results. One way to streamline your checkout experience is to integrate that choice (Guest, Login or Register) at the top of the checkout page itself, and gray out the rest of the page until they choose. With this strategy, that choice is still an additional step, it just prevents from having to load a different page.

Another strategy is to open the checkout page directly, and have a guest checkout by default. Simply add a link at the top for them to login or register.