If an adopter is not with you in person, we highly recommend using the credit card checkout because that flow puts the e-signature functionality on their phone.
If the adopter does not have a smartphone: The checkout can also be sent via email by clicking "customer has no phone" (beneath the field with the phone number). You can then copy/paste the URL of the new tab that pops up into an email to the adopter.
Here is a video screencast showing you how to email the link to an adopter:
You can read more about processing transactions here: How do I process adoptions (and other transactions)?