These are E-Gift Certificates. You can send them by email to whomever you choose. No physical or paper certificate is issued or needed for redemption.
Gift certificates can be purchased in increments of $10. For example if you wanted to send 2 people $20 each, you would simply purchase a quantity of 4. Giving you a total of $40.
Once purchased the total amount will be available in your account. You can then send any amount or portion of your total to 1 or more recepients by email.
See the Gift Certificate FAQ link under "Information" for more information on buying and using Gift Certificates.